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Anyone else doing brake proportioning valves? by Raydar
Started on: 02-08-2015 01:25 PM
Replies: 36 (1496 views)
Last post by: ag9123 on 05-05-2018 10:46 PM
Raydar
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Report this Post02-08-2015 01:25 PM Click Here to See the Profile for RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
My 88 has a very pronounced front-bias to the brakes. From talking with other folks, I understand that this is pretty typical.

This was aaron88's mall thread.
//www.fiero.nl/forum/Forum4/HTML/064690.html
It would appear that he no longer is making these for whatever reason.

I was wondering if there is another solution for this situation, or if someone else might be inclined to produce these.

I would have posted this request in the mall, but there are obviously technical aspects to this that are more involved than simply posting a "want to buy" thread.

Thanks.

------------------
Raydar
88 Formula IMSA Fastback. 4.9, NVG T550

Praise the Lowered!

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Gall757
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Report this Post02-08-2015 01:44 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
Don't miss the Better Braking thread over in General Chat. It covers a bunch of issues including front bias (when it's not off-topic about modern cars ).
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Report this Post02-08-2015 01:48 PM Click Here to See the Profile for RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
Cool. Thanks.
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Report this Post02-08-2015 02:43 PM Click Here to See the Profile for cmechmannSend a Private Message to cmechmannEdit/Delete MessageReply w/QuoteDirect Link to This Post
Has anyone made up a data base for bias over years.
I haven't had much exposure to 88s. This is also going to have variables with drive train, suspension, condition.
To me it seemed as follows:
84 heavy rear.
85 heavy front.
86 neutral, but wide variables.
87 tad heavy rear.

It boils down to finding that happy medium. Seems the best would be heavy rear until the rear unloads from heavy cornering and panic stops. Had my share of a little wet, light turn, hit the brakes, goes straight.
Has anyone installed rear weight differencial valves?


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Report this Post02-08-2015 02:49 PM Click Here to See the Profile for jmbishopSend a Private Message to jmbishopEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Gall757:

Don't miss the Better Braking thread over in General Chat. It covers a bunch of issues including front bias (when it's not off-topic about modern cars ).


It's a great thread but this is a different issue. Changing tire size and running a staggered setup changes things and the only way to fix it is by changing wheel and tire sizes otr tuning the bias to your setup.
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Report this Post02-08-2015 03:09 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
If the proportioning valve is different for different years, I have not heard of it. People swap them all the time, and think nothing of it. It's more likely that the adjustment mechanism in the rear calipers is not working, and what little stopping power that is assigned to the rear by the proportioning valve goes unused as the rear adjuster fails.

edit...The Ogre is going to update his cave about rear e-brake adjusters...apparently GM has blown a lot of smoke about them over the decades.

[This message has been edited by Gall757 (edited 02-08-2015).]

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Raydar
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Report this Post02-08-2015 04:07 PM Click Here to See the Profile for RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Gall757:

If the proportioning valve is different for different years, I have not heard of it. People swap them all the time, and think nothing of it. It's more likely that the adjustment mechanism in the rear calipers is not working, and what little stopping power that is assigned to the rear by the proportioning valve goes unused as the rear adjuster fails.

edit...The Ogre is going to update his cave about rear e-brake adjusters...apparently GM has blown a lot of smoke about them over the decades.



My adjusters (and parking brake) work fine. My brakes work fine, in general, except for the front bias, under hard braking.

I had a conversation with fieroguru. He seemed to share my opinion that the 88 brakes are biased towards the fronts. I asked him about increasing my front tire size. His opinion was that it probably wouldn't help much. Would just bandaid the issue, and probably not that well. (That's what I took away from the conversation, anyway. Not trying to put words in his mouth.)
Aside from that, 88 GTs and Formulas (which mine is) started out with staggered tires, anyway.

I have not explored aftermarket proportioning valves.

[This message has been edited by Raydar (edited 02-08-2015).]

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olejoedad
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Report this Post02-08-2015 04:12 PM Click Here to See the Profile for olejoedadSend a Private Message to olejoedadEdit/Delete MessageReply w/QuoteDirect Link to This Post
88 proportioning valve is different. 88 caliper bores, front and rear are the same. 84-87 caliper bores are different sizes, f/r.
I use an 88 valve when doing 4 wheel G/A swaps.
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Report this Post02-08-2015 07:05 PM Click Here to See the Profile for LarryinkcSend a Private Message to LarryinkcEdit/Delete MessageReply w/QuoteDirect Link to This Post
WCF and Aaron both suggested disabling the 88 proportioning valve for the C4 brake mod on my 88 Mera.

Here's the explanation:

It looks like I have some bad news for you. By my calculation you will have about 5097 lb (at max braking effort), front caliper force and only 1266 lb rear caliper force with the stock proportioning system. That's only 20% on the rear system. Depending on how soft your tires are you will need about 40% of your system force at the rear caliper in order to make use of my proportioning adapter. If you completely remove the proportioning system you still only have about 2825 lb at the rear caliper. That's 36%

If you use the stock 88 rear caliper (which is about 45.5mm if I remember right) then you could get 42% on the rear, but that's only if you remove the proportioning. But even with my adapter installed and cranked all the way in you will be in abouts 2394 lb, which is 32% rear braking. Unless you are running slicks that's not enough. and running 42% (no proportioning) might be too much rear, therefore causing a probelm with the back end swinging out.

There is a big problem with brakes when using a modern ABS designed system. They are basically designed to fail safe (if the abs module stops working) and are therefore always undersized on the rear. I'm not sure what to advise here. You could try installing your calipers as purchased and remove the proportioning system rubber regulator (not sure what to call it). And just see how the balance is. Make sure the fronts lock up first.

If I sold you a proportioning adapter you would just crank it to full and still end up with too much rear proportioning (with my adapter installed the system will start proportioning the rear at about 800psi system pressure, if fully cranked).

Sorry for the bad news, but your brakes will be much better than stock anyway. I'm just afraid you haven't left any room for proportioning. However there are a lot of other factors that come into play such as front to rear tire to ground contact patch, tire compound, and driving style. It's too hard to calculate before hand which is why we like to have adjustable proportioning. I'm not sure what to recommend in your case though. You would basically need a rear caliper that measures 47 mm to 52 mm I think. Nothing larger but maybe 46 would work (that's stork Vette rear).

Maybe what I would try first is using your stock 88 calipers on the rear, and remove the proportioning. Then go from there. This would give you a gauge to start from.

When you disable the proportioning in you car. Do not remove the spring. Instead remove the rubber washer looking seal at the end of the plunger (not the shaft o-ring seal).


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Report this Post02-08-2015 07:24 PM Click Here to See the Profile for fieroguruSend a Private Message to fieroguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
As you stagger tire width to better match weight bias front/rear, you will want more rear brake bias.
If you run shorter wheels/tires up front and taller in the rear, you will want more rear bias.

The C4 world has stiffer springs they can swap into their combo valves to increase rear bias, but Aaron88's setup is the only one I am aware of that provides linear adjustment. How it works is the combo valves start with the front/rear brakes seeing close to the same pressure at very low pedal pressure. At the "Knee Point" the combo valve starts to reduce the rear pressure as it assumes the cars weight is shifting forward. What the C4 stiffer springs and Aaron's adjuster do is move the knee point to a much higher pressure - so the rear line pressure remains closer to the front line pressure up to a much higher pedal input pressure.

All off the shelf brake bias adjusters can only reduce rear brake pressure (and therefore bias), not increase it.

The oddity of the Fiero is the combo valve controls 100% of the bias front/rear because it uses the same diameter caliper pistons front year. So to create any bias, it must limit the rear brake line pressure.

Most other cars use smaller caliper pistons in the rear which builds in less rear braking even with the same line pressure as the front. So the rough bias is due to caliper piston diameters, then it can be fine tuned within the combo valve with line pressure reductions. A combo valve from one of these other applications will likely result in more rear brake line pressure, which will increase rear bias substantially, then you probably could dial it back in with an off the shelf bias adjuster to reduce rear brake line pressure.

So you can better "see" what I am talking about, here is a chart where I used an air cylinder to apply various levels of pedal pressure and measured the caliper line pressure in the front and rear on an 88 2.5L Fiero with the engine running (stock brake booster). As you can see, the rear line pressure starts out at 75% of the front pressure at very low pedal pressure and drops to around 56% at 30 PSI input pressure and stays there as pedal pressure continues to increase. Since the caliper pistons are the same front/rear (48mm = 1809 mm^2) and the brake rotors are the same front/rear, this pressure difference front/rear is the brake bias front/rear as well.

code:

Caliper Line Pressure Comparison Stock 88 Booster

Input Air 2.5L 2.5L Rear vs.
1 ½” air cylinder Front Rear Front
(psi) (psi) (psi)
5 n/a n/a
10 400 300 75%
15 600 400 66%
20 800 500 63%
25 925 600 65%
30 1200 675 56%
35 1225 700 57%
40 1250 700 56%
45 1300 725 56%
50 1325 725 55%
55 1375 775 56%
60 1400 775 55%





Now let's look at an 88 Corvette (non-HD, so it uses the same diameter rotors front/rear)...
Front Caliper: Dual 38mm pistons = 2267 mm^2
Rear Caliper: Single 40mm piston = 1256 mm^2

So with the exact same line pressure front/rear the rear calipers will have 55% less braking force than the fronts and end up with about the same final bias as the Fiero (unless its combo valve further reduces rear line pressure), but does so via caliper size vs. reducing line pressure.

Unfortunately the combo valve for the C4 Vettes is built into the brake master cylinder, so you can't just swap it into a Fiero. Plus the C4 brake master cylinder bore is smaller and the line fittings are true metric vs. the fiero ones being standard... so it isn't a good option for a retrofit (but is a good example showing how things are different). Just as an FYI, the C4 world can buy stiffer springs for their combo valve to increase rear bias as well, but those are not adjustable like Aaron88's).

It would be a fun experiment to measure caliper line pressures on several GM cars with similar style combo valves in an effort to find one that sends more pressure to the rear than the stock fiero ones do... Once you have too much rear line pressure, reducing it is quite simple with many off the shelf options to pick from.

EDIT: Just saw Larryinkc's post...

From my example above, the stock 88 rear bias limits at the high end with the rears seeing about 56% of the braking that the fronts do.
The C4 setup with the SAME line pressure to the front & rear will have the rear brakes at 55% of the fronts, which is very, very close to the 56% seen with the stock brake setup. The easiest way to ensure front/rear see the same pressure is to eliminate the combo valve, and with it gone the brake bias will be very close to stock, but you lose the additional rear bias at lower pedal inputs (the function of the combo valve).

[This message has been edited by fieroguru (edited 02-08-2015).]

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Report this Post02-09-2015 02:34 PM Click Here to See the Profile for tsharkSend a Private Message to tsharkEdit/Delete MessageReply w/QuoteDirect Link to This Post
...

[This message has been edited by tshark (edited 09-08-2018).]

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Report this Post02-09-2015 05:43 PM Click Here to See the Profile for RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thanks for jumping in, Paul.

It's really easy for me to speak in generalizations. ("My back brakes suck.")
You can explain it so that it makes sense.

[This message has been edited by Raydar (edited 02-09-2015).]

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Report this Post02-09-2015 06:27 PM Click Here to See the Profile for doublec4Send a Private Message to doublec4Edit/Delete MessageReply w/QuoteDirect Link to This Post
I plan on reproducing these. Right now I only have the 84-87 I've made drawings / models for. I'm getting them quoted right now.

I would need one of the 88's so I can do the same.
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Report this Post02-09-2015 06:38 PM Click Here to See the Profile for E.FurgalSend a Private Message to E.FurgalEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Raydar:

My 88 has a very pronounced front-bias to the brakes. From talking with other folks, I understand that this is pretty typical.

This was aaron88's mall thread.
//www.fiero.nl/forum/Forum4/HTML/064690.html
It would appear that he no longer is making these for whatever reason.

I was wondering if there is another solution for this situation, or if someone else might be inclined to produce these.

I would have posted this request in the mall, but there are obviously technical aspects to this that are more involved than simply posting a "want to buy" thread.

Thanks.



is this a stock car,??
weight change(per wheel have car scaled), spring rates, ride height, shock valving, all effect this.. as the stock settings "assume" the way the vehicles weight will be transferred from/to each wheel and how fast that weight moves,
so is the car seeming having to much front bias, from changes that upset the engineered weight transfer that the stock bias was ment for?

[This message has been edited by E.Furgal (edited 02-09-2015).]

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Report this Post02-09-2015 07:58 PM Click Here to See the Profile for Steven SnyderClick Here to visit Steven Snyder's HomePageSend a Private Message to Steven SnyderEdit/Delete MessageReply w/QuoteDirect Link to This Post
If aaron88 can't make one soon enough for you, this is another option:

JEGS Adjustable Proportioning Valve and Distribution Block

555-63025

It replaces the entire stock proportioning valve / distribution block with an adjustable unit. You will need to make up the lines from the master to the block and/or use some adapters for the rest. The only downside of this one is you lose the safety shuttle valve that blocks flow to one circuit (front or rear) if there's a big leak. The advantage is I think you can increase proportioning more than you can with aaron's setup.

I have had one on the shelf for a few years that I never ended up installing because I got one of aaron's direct-swap adjustable valves. If anyone is interested feel free to PM me, I'll let it go for cheap. They're almost $100 new.

[This message has been edited by Steven Snyder (edited 02-09-2015).]

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Raydar
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Report this Post02-11-2015 12:23 AM Click Here to See the Profile for RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by doublec4:

I plan on reproducing these. Right now I only have the 84-87 I've made drawings / models for. I'm getting them quoted right now.

I would need one of the 88's so I can do the same.


Cool. I'll keep an eye out for these, when you have them.

And thanks, Steven.
That is an option, but I don't have the capability to fabricate lines. Would like to explore the simpler option first, if it becomes available.

 
quote
Originally posted by E.Furgal:

is this a stock car,??
weight change(per wheel have car scaled), spring rates, ride height, shock valving, all effect this.. as the stock settings "assume" the way the vehicles weight will be transferred from/to each wheel and how fast that weight moves,
so is the car seeming having to much front bias, from changes that upset the engineered weight transfer that the stock bias was ment for?



It's far from stock, but I have attempted to not do anything that would upset the balance. All changes were... in proportion to one another.
It still was heavily biased towards the front, even before I made any changes, except that the stock Formula rubber was quite hard. I could lock up the fronts even more easily.
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Raydar
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Report this Post04-18-2018 05:59 PM Click Here to See the Profile for RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
I think I may have found a source for the 88 combi valves.
Well...three of them, anyway. If it works out, I'll publish the source for the other two.
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Report this Post04-18-2018 07:07 PM Click Here to See the Profile for olejoedadSend a Private Message to olejoedadEdit/Delete MessageReply w/QuoteDirect Link to This Post
Dumb question....
Has anyone investigated using a bias valve from a FWD car (pre ABS) on a Fiero?

Maybe there is an off the shelf solution available that we haven't thought of.....
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Report this Post04-18-2018 08:32 PM Click Here to See the Profile for pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
Most FWD cars are diagonally split, so they'll have two proportioning valves; one per rear brake. That's probably not what you want for a Fiero, unless you plan on a diagonally split system.

The bigger issue is what junkyard parts are not normally adjustable.

Anything you'll find on Jegs/Summit will be "off-the-shelf", and adjustable, and thus recommended for a homebrew braking system. If I had to guess, I'd guess that the OEMs initially tuned a car with an adjustable valve, and then once they found the right setting, they had a fixed version built. But starting with something fixed strikes me as the wrong way of doing things.

For "cheap" and "off-the-shelf", at the expense of fast tuneability, why not try an assortment of springs to replace the stock spring? For small quantities, I have purchased from:
https://www.centuryspring.com
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Report this Post04-19-2018 03:56 PM Click Here to See the Profile for RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by pmbrunelle:

....
For "cheap" and "off-the-shelf", at the expense of fast tuneability, why not try an assortment of springs to replace the stock spring? For small quantities, I have purchased from:
https://www.centuryspring.com


It's not just a matter of tuneability. I have a situation, now, where I think the combination valve is causing problems. When I nail my brakes, the left front wants to lock up long before the others.
I have replaced the lines, rotors, pads and calipers, and bled the system to a fare-thee-well, with exactly zero change.
I thought it might all be my imagination, until I bought another 88, and found that the brakes work much better - and relatively evenly. The combi valve is about all that's left.
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Report this Post04-19-2018 07:21 PM Click Here to See the Profile for pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
The combination valve doesn't do anything with the front circuit other than split the line towards the left and right brakes.
Hydraulic pressure between both sides is (supposed to be) the same.

I would start by investigating the amount of weight on each tire.

If the front tires don't share the weight of the front end evenly, then the side supporting less weight will lock up prematurely.

This may be because you shifted weight around, or maybe you have cut springs that are not the same height/rate on both sides? If one spring is shorter, then there will be less weight on the tire having the shorter spring.

Do you have scales? I don't, so what I do is I drive to truck weigh stations off the freeway at night when nobody is there. Be sure to stay in the car while being weighed.

[This message has been edited by pmbrunelle (edited 04-19-2018).]

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Report this Post04-19-2018 08:02 PM Click Here to See the Profile for PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by pmbrunelle:

what I do is I drive to truck weigh stations off the freeway at night when nobody is there.


I didn't know that was possible. Are the scales always "on"?

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Report this Post04-19-2018 09:22 PM Click Here to See the Profile for pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Patrick:

I didn't know that was possible. Are the scales always "on"?


Here on the Québec autoroutes I travel on, yes, the scales are always left on. I can't say how it is in other jurisdictions though.
Not sure about the legality either, as there are "cars prohibited" signs at the entry point, but I figure that stopping momentarily at a closed weigh station isn't going to bother anyone.

There are flashing lights to indicate when trucks must stop for inspection; I avoid the station during these times, to not interfere with their operations.

The weighing platform is composed of multiple independent sections. It's arranged to measure the weight of each axle (or axle pair) on a big truck.

There are LED displays that are always on, and visible from the vantage point of the driver on the vehicle being weighed.
The display shows the weight per axle, and the total of all axles.

To figure the left/right distribution, you would have to drive with one side of car on pavement, and the other side on the scales, measuring half the car at a time.
Repeat for the other side.
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Report this Post04-19-2018 10:13 PM Click Here to See the Profile for La fieraSend a Private Message to La fieraEdit/Delete MessageReply w/QuoteDirect Link to This Post


I didn't have a problem with brake bias but for faster stopping and to make the car rotate better at very short radius turns I installed one of these.
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Report this Post04-20-2018 08:05 AM Click Here to See the Profile for wftbSend a Private Message to wftbEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by La fiera:



I didn't have a problem with brake bias but for faster stopping and to make the car rotate better at very short radius turns I installed one of these.


So is your stock prop valve disabled? Is that additional valve on the front circuit or the rear circuit?
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Report this Post04-20-2018 06:37 PM Click Here to See the Profile for La fieraSend a Private Message to La fieraEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by wftb:


So is your stock prop valve disabled? Is that additional valve on the front circuit or the rear circuit?


Yes, I disabled the rear circuit and use the adjustable prop. valve to adjust the pressure to the rear.

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Report this Post04-26-2018 06:04 PM Click Here to See the Profile for SP1200Send a Private Message to SP1200Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by La fiera:



I didn't have a problem with brake bias but for faster stopping and to make the car rotate better at very short radius turns I installed one of these.


Can I please have the details of this setup?
Is there a thread about it? write up? Part numbers ect?
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Report this Post04-26-2018 11:22 PM Click Here to See the Profile for wftbSend a Private Message to wftbEdit/Delete MessageReply w/QuoteDirect Link to This Post
The answer to my question did not make sense to me but i let it slide. You cannot disable the rear circuit of the prop valve. You disable the whole prop vave or you leave it alone.
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Report this Post04-27-2018 02:52 PM Click Here to See the Profile for La fieraSend a Private Message to La fieraEdit/Delete MessageReply w/QuoteDirect Link to This Post
I've been looking for the information I used to do it but can't find it. If I remember correctly I modded the rear circuit so it can have full pressure and then installed the Jeg's proportioning unit in line. I'll keep looking.
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wftb
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Report this Post04-27-2018 04:30 PM Click Here to See the Profile for wftbSend a Private Message to wftbEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thanks
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SP1200
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Report this Post05-01-2018 10:30 PM Click Here to See the Profile for SP1200Send a Private Message to SP1200Edit/Delete MessageReply w/QuoteDirect Link to This Post
yea never got an answer to my question ether. I hate how that keeps happening.
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wftb
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Report this Post05-02-2018 12:51 PM Click Here to See the Profile for wftbSend a Private Message to wftbEdit/Delete MessageReply w/QuoteDirect Link to This Post
Here is some good reading on the subject of the prop valve. It helped me a lot to understand how it works, how to mod it and disable it. We are a smallish group of people on here and most of us are not here every day. Sometimes questions just don't get answered, and a lot of threads just die without any resolution. The nature of the beast nowadays.
//www.fiero.nl/forum/Forum2/HTML/109173.html

If you look at the drawing on pge one, second drawing down you will see the plug and spring and if you look carefully you will see a rubber ring 'flapper' style valve drawn in blue. Removing that rubber valve and putting the prop valve back together turns the prop valve in to a fluid junction and you should get a 50/50 brake distribution front to rear. I did this on my setup and the right front locked up same as before. That told me that I needed to send more than 50% pressure to the rear brakes. Since aftermarket prop valves are primarily designed to reduce pressure, then putting one on the rear brake circuit would increase braking to the front circuit.The best I could get would be a 50/50 split and I proved that was not good enough for my set up. So I put my Wilwood valve on the front circuit and now I have the brakes I have always wanted. It is a mistake to call these aftermarket valves combination proportional valves because even if you run all circuits in and out of the valve, there is no interaction between the front and rear circuits. If you look at a pic of my setup (last pge of above thread I think) you will see 2 empty holes on the valve body.

[This message has been edited by wftb (edited 05-03-2018).]

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ag9123
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Report this Post05-04-2018 12:29 PM Click Here to See the Profile for ag9123Send a Private Message to ag9123Edit/Delete MessageReply w/QuoteDirect Link to This Post
So it looks like there are 3 different proportioning valves on the Fiero per Ogre:
I'm just grabbing some info from some previous posts...

Kudo's to Fierobsessed for putting a lot of info out there.
//www.fiero.nl/forum/Forum2/HTML/109173.html

"Prop valve part numbers:
84 exc Y82 10026021 (not sure on spool diameter/color)
84 w/ Y82 10036563 (RPO Y82 is Merchandised Pkg, Fiero = Fiero Indy) 0.286" spool (gold)
85-87 10036563 0.286" spool (gold)
88 10071501 1/4" diameter spool (red)"

The 84-87 brake caliper pistons:
49mm FT
47.75mm RR

The 88 brake caliper pistons:
47.75 FT
47.75mm RR

Ignoring the proportioning valve for a moment, it would appear that the 88 has a slight FT bias (based solely on caliper piston area alone)
Per Fierobsessed:
Based on spool diameter:
0.20" 84% above 477 PSI
0.25" 75% above 306 PSI (My 88GT Fiero) RED
0.286 67% above 233 PSI (My 84 Indy Fiero) GOLD
0.30" 64% above 212 PSI
0.35" 51% above 155 PSI

"So if you wanted more rear brake bias, you would need a narrower spool, and vice versa. The spring's cracking pressure also changes greatly with the different spool sizes, so I listed them as well, assuming the spring delivers about 15 lbs of force."

The 88 appears to be using the .25 prop valve spool to increase rear brake bias at higher brake pressures vs the .286 spool in the Indy/85-86

In keeping with Pontiac's design theory, I would think an 88 prop valve on a GA FT/RR setup would restore braking bias to stock (88) specs..

Does this sound about right?

Stock for stock, do the 88 brakes actually perform better than the 84-87 brakes (shorter stopping distance) ,vented rotors aside?

------------------
1984 Indy Fiero

[This message has been edited by ag9123 (edited 05-04-2018).]

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wftb
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Report this Post05-04-2018 08:26 PM Click Here to See the Profile for wftbSend a Private Message to wftbEdit/Delete MessageReply w/QuoteDirect Link to This Post
According to every test and almost everything that has been posted on here the 88 brakes stop shorter and are more fade resistant than 84-87 brakes.
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Report this Post05-04-2018 09:17 PM Click Here to See the Profile for cvxjetSend a Private Message to cvxjetEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have recently finished swapping my 85 Fiero to 88 brakes (Rear subframe/suspension/brake swap, and front 88 calipers with Sluppy brackets/hubs and 12" vette discs) I grabbed several prop' valves from the JY...I would swear that an 87 had a red spool but a different spring from the 88 I grabbed....I ended up with a red spool and the softest spring so that the prop' valve starts working earlier......Before, stock 85 gold spool, very front-biased, with red spool but strong spring somewhat front-biased....Now, I seem to have come very close to proper balance.....

(By the way, the 88 brakes have better fade resistance simply because they are...Ventilated discs!)
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fieroguru
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Report this Post05-04-2018 09:30 PM Click Here to See the Profile for fieroguruSend a Private Message to fieroguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by ag9123:
Stock for stock, do the 88 brakes actually perform better than the 84-87 brakes (shorter stopping distance) ,vented rotors aside?


The two largest differences are:
88's have a larger rotor diameter (10.43 vs. 9.69), so their calipers have more leverage to stop the car.
88's are vented so they are more fade resistant.
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Report this Post05-05-2018 10:46 PM Click Here to See the Profile for ag9123Send a Private Message to ag9123Edit/Delete MessageReply w/QuoteDirect Link to This Post
Ah, I didn’t know the 88 had larger rotors...
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