Could someone explain to me how to use and install a proportioning valve on a Fiero? I've been reading different threads in the archive and now I'm just more confused. (doesn't help it's late)
I understand brake bias is set on either a brake dyno. (what does that look like anyways), or by carefully watching which wheel locks up first on a dry pavement. < Does the latter still apply on a wet road?
Remove the plug from the rear of the original Fiero combination valve, remove the plunger and spring and either braze or weld closed, the small hole that is under the small plastic cap in the center of the plug.
Here's a pic of the wilwood prop valve installed, can't recall what size the fittings are though.
I'm interested in this in a more general sense, including trying to understand how the Held (or any aftermarket) setup works, and because of the possibility of adapting other Big brakes to the Fiero. (Fieroaddiction, Subaru, Porsche, etc.) From what I understand the brake bias would be off from the stock Fiero bias. Usually a proportioning valve is used to fix that. How does it get plumbed in though? And why can't I get a metric proportioning valve to avoid a slew or problems? Is there a way to completely skip that copper thing? I don't trust my welding skills (I have none) to weld up a fitting. Especially if I don't understand what it does.
Could someone give me a primer to explain all of this?
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04:15 AM
PFF
System Bot
Aug 2nd, 2009
Austrian Import Member
Posts: 3919 From: Monterey, CA Registered: Feb 2007
You need to adjust the braking bias because as the brakes are applied, the weight of the vehicle is shifted to the front wheels. When this happens, approximately 70 percent of the vehicle's weight is on the front wheels and 30 percent is on the rears. Generally, braking systems compensate for most of this weight shift in the brakes themselves. As you have noticed, the front brake rotors are usually a few inches larger in diameter than the rears. The front calipers are usually multipiston design, and the rears have much smaller friction pads and calipers. Fine-tuning the brake balance is where the adjustable proportioning valve comes into play. The valve is installed in the brake line going to the rear wheels. You want to apply as much pressure to the front brakes to get maximum braking work. To adjust the valve you want to go out on a very safe road where you can do multiple aggressive stops (50-0 mph). You will want to do braking runs that are called threshold braking. This is just shy of locking up the brakes. The valve is adjusted properly when the rear brakes won't lock under maximum threshold braking until you get to about 10 mph. When I say lock, I mean that you want the right rear wheel to just lightly skid. Obviously, you will need to do this in dry conditions, on clean asphalt, and again, in a safe place.
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10:10 AM
aaronkoch Member
Posts: 1643 From: Spokane, WA Registered: Aug 2003
The fiero is a unique car as far as the brakes go, there are 2 circuits: front and back. Most FWD cars will have the circuits feeding front-left & rear right, and the second front-right and rear-left. The factory proportioning valve in the fiero limits maximum pressure to the rear hydraulic lines, while still allowing a quick take-up fluid push to the rear to make sure the rear brakes get enough.
If I remember well, when I did my kit, the kit was made by Ryanne (rights bought by Held). They said to leave the plunger, but remove the spring, then braze shut. I really can't remember for sure, but I believe it was to keep the safety feature of the factory valve, while removing any pressure limiter. My brake light still worked correctly after the swap.
The Wilwood adjustable valve works in a similiar way to the factory valve, only it works as a straight pressure reducer. So the knob sets the output pressure to a percentage of the input pressure. It is plumbed inline with the rear brakes feed off the factory (modified) valve.
It is somewhat tricky to get set right, here's what I did:
Set brake bias to max front, so set bias valve to minimum flow (I believe the wilwood device only limited pressure to 50% of input at max) Get entire car done as you will be driving it, wheels, brakes, tires, swaybars, etc. Anything that will affect handling or grip. Find a large parking lot that you can get wet and play on. Get a bucket, and empty a bottle of dawn dish soap into it, fill the rest with water. Wet parking lot with hose, then pour bucket out, careful, it's like ice. Slowly drive thru wet area with windows down and apply brakes.
-If fronts lock before rears, adjust a slight bit more rear bias, and repeat.
I would do this until the rears would lock at the same time as front, then I would reduce rear pressure by 1 turn. When you really romp on the brakes, there is a LOT more weight transfer than on a wet parking lot, and you REALLY want the fronts to give first. (or at least I did for safety) I would do a full lock-up test on dry pavement when you think you're done setting it.
Also, once you have it dialed it, keep track of where it is, so when you adj. for racing it or stunt driving (don't ask, it was fun) you can put it back quickly (for a quick getaway.. )
Hope that helps..
[This message has been edited by aaronkoch (edited 08-02-2009).]
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11:39 AM
aaronkoch Member
Posts: 1643 From: Spokane, WA Registered: Aug 2003
As an aside, I remember thinking after setting it that I was surprised how much the rear brakes were used in the fiero.. WAY more than on other cars I'd driven..
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11:41 AM
Aug 4th, 2009
Austrian Import Member
Posts: 3919 From: Monterey, CA Registered: Feb 2007
As an aside, I remember thinking after setting it that I was surprised how much the rear brakes were used in the fiero.. WAY more than on other cars I'd driven..
Yes, I've been told that it's nearly 50/50 from the factory. Just slightly less rear. I think it's because of the mid-engined design, and weight distribution. (what kills this theory though, is seeing brake setups of Porsche Boxter, Ferrari, Lamborghini, Lotus, etc. - they have bigger front calipers.) Maybe it has something to do with almost equal size front and rear tire widths. (most exotics are staggered)
Thank you again for the responses. Please keep the explanations coming.
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12:38 AM
IFLYR22 Member
Posts: 1775 From: Tucson, AZ. Registered: May 2007
The brake systems are all the same in the end. The proportioning valve on GM's is a dual function device. It turns on the light if there is a lack of fluid in either of the two halves of the master cylinder. The plunger grounds the wire. The front circuit is, essentially, a one input, two output splitter. the rear half is just a pass through. the imbalance of pressure due to a lack of fluid on one side of the plunger causes the plunger to shift and ground the wire. you don't need this device at all. the front circuit can be replaced with a "T" fitting. The rear circuit can just be connected to the master cylinder. You won't have an idiot light like this, but... To adjust the biasing, though, you need a proportioning valve in the rear circuit to limit the pressure in relation to the front. The front should always get full pressure. keep in mind, the pressures can reach up to 1600 psi.
when adjusting the pressures, I use two gauges. one on the front circuit, one on the rear circuit, post proportioning valve. This is overkill for some people, but it does allow exact balancing.
You don't want a left / right circuit system. If one circuit is lacking fluid, then only one side is getting usable pressure. In a sudden stop with minimal fluid, one side will work better and cause a brake steer situation. There are racing types (and aircraft) where a L / R circuit system is desirable, but not on a road use car.
Cross feed systems are relatively new systems, but even new front engine, rear wheel drive cars still use the old design system (with limited exceptions).
-Dave
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01:28 AM
Fierobsessed Member
Posts: 4782 From: Las Vegas, NV Registered: Dec 2001
I had an opportunity to study and understand how the fiero's brake system really works. I think there are a lot of myths about how it works, only a few people really understand what the combination valve does, let alone the master cylinder. I don't know if I fully understand it all yet, but I would like to provide some insight.
First is the Master cylinder. I'm not going to go into detail about the internal workings, but I will say that the master cylinder has only one diameter bore that it uses to activate the brakes, but it has a larger bore used to get the pads to react quicker (quick take up). Because it only uses one bore diameter, the pressure on the output ports is the same. Since it is a balanced pressure master cylinder, if the fronts loose pressure, the rears will as well, and vice versa. So what they did is make a last ditch effort, that if one part of the master looses pressure, the last 1/3'rd (I am approximating) of pedal travel is reserved for the other functioning side. It's an interesting arrangement, but very safe.
Second, the combination valve. This valve is often mistaken for a "Proportioning valve" which in our cars, it is most definatly not. It's called a combination valve because it has two functions.
It holds off when applying the front brakes till the rears begin to build pressure. This function is called metering. Its supposed to help straighten out the car using the rear brakes then apply the fronts with the additional weight transfer. This function is only really meant to be utilized with DRUM BRAKES since they have to be "taken up" before they function. However, for some reason our Fiero's do retain this function. Its not quite as dramatic as it is with drum brakes but the function is still present. This function is done by the piston that is under the cap with a spring within the combination valve. Some people opt to remove this function, which makes a difference in how the car brakes. It is not unsafe, but it isn't something GM wants you to know. They remove the piston, and the spring, then plug the vent hole on the cap, most weld it.
The other function of the combination valve is safety. There is a spool (basically a piston that is used as a valve) inside the combination valve. It has two same size bores on it. Normally the piston sits neutral in the middle of the combination valve since the pressures coming from both the ports on the master cylinder to both the front and rear are equal. If for any reason, one side looses pressure, this will cause an inbalance on the two sides of the spool, forcing it to one side. This will either cut off the front brakes or the rear, causing the remaining system to retain functionality. This also turns on the "BRAKE" light.
As far as proportioning. There is no such thing on the Fiero. Both fronts and rears recieve the same pressure. What makes all the difference is the diameter of the pistons in the caliper. Pressure multiplied by effective area = braking force. It's as simple as that.
What are the Fiero caliper bore diameters anyway? I know the rears are smaller, as per normal.
Let me know if you think any of this is wrong. I am 98% sure this is how the Fiero's and GM brakes in general work. I hope this helps you make an informed decision about a proportioning valve.
Looking at the pictures you posted, the proportioning valve is only interrupting the front brake feed from the master to the combination valve. To me, this doesn't seem right, as it would cause an imbalance in the combination valve, resulting in the rears getting shut off.
When installing a Proportioning valve, Most people remove the Combination valve, T off the front supply directly through the front calipers, then put the proportional valve inline from the master to the rear brakes. This is how it is intended to be used.
If you leave the combination valve in place, you would be much better off getting a proportioning valve that has 4 ports on it, an in and out for both the front's and rears. This way you retain safety and have an adjustable BALANCE of pressure from front to rear.
"Proportioning" Valve are Not the only reason in brake bias.
OEM PV only work for panic/emergency stop. Does nothing in normal use.
Real/natural bias in the rest of the Brake System. Usually smaller rear brakes. Can be small pad, rotor, caliper piston, and/or other thing. See Cave, brake upgrade.
Use an aftermarket PV, like GA "upgrade," is a time bomb. If aftermarket PV fail or miss adjustment, you not know until it you car ends up sideways or worse. Baer, Wilwood, etc, do not like this for it reason.
Combination valve Disk/drum Com Valve as 3 function. Hold valve for front brake balance spool P V
Disk only, like Fiero, or drum only Do Not contain any Hold Valve. They have only balance spoon and PV.
{Source image: WEM/lawrence mazza }
quote
Originally posted by Austrian Import: I understand brake bias is set on either a brake dyno. (what does that look like anyways), ....
No. Bias not that simple. Some thing yes, and some no. Can write a book or two.
What looks? You drive on and step down on brake. Computer connect to 4 pad under the cars test it. DE inspection use it. Not much to see.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
http://forums.evolutionm.ne...-car-brake-bias.html < -- interesting discussion. I'm not sure I agree so far: Different temperature ranges would make for some rather unpredictable behavior. (i.e.: the car will stop differently when brakes are cold, and differently when brakes are heated. - not sure I want to find out on a track, or on the street)
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08:35 PM
Austrian Import Member
Posts: 3919 From: Monterey, CA Registered: Feb 2007
Originally posted by theogre: No. Bias not that simple. Some thing yes, and some no. Can write a book or two. What looks? You drive on and step down on brake. Computer connect to 4 pad under the cars test it. DE inspection use it. Not much to see.
Like I said, I'm starting to understand the basics of it. I'm far from understanding it enough to try it.
So it looks like a regular dyno. Is it known by another term? We don't use inspections in California. (They don't care if your car stops safely, all they care about is emissions. I never understood that. ) Are brake dynos known by another term? I would like to search for one locally to me so I can have them test a setup for safety and correct bias. (assuming there is a way to stationary test it, other than going 50mph-0mph and risking life an limb)
No it not will regular dyno. Think 4 small scale. They measure brake instead of weight. The load cell measure sideway instead up/down like weight.
Not many people have one. De nobody have one except inspection lane. Another name? no idea.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Originally posted by Fierobsessed: Aah, heres what you want! It replaces the combination valve with a new functional proportioning and distribution block, with the safety!! http://www.jegs.com/p/JEGS/...lock/762853/10002/-1
Of note, this valve block can only reduce the rear brakes, if you desire more rear braking capacity, you will need different calipers.
Thanks. This looks like what I need. Also looks much better than the Ryane/Held setup I started this thread with.
I reposted the photos and info for faster reference:
quote
JEGS Adjustable Proportioning Valve and Distribution Block
Designed for dual bowl master cylinders and incorporates an adjustable proportioning valve and a distribution block into one unit. Precision machined from billet aluminum and available with a black anodize or polished finish. Each style features the distinctive JEGS logo. Features a rotary adjustment valve with directional arrows for fine incremental adjustments to reduce or increase the front to rear bias, two inlets from the master cylinder, two outlets to the front brakes and 1 outlet to the rear brakes to distribute fluid to all four wheels. Includes five stainless steel fittings for 3/16" brake line, brake light switch and wiring pigtail. Conveniently mounts in any location.
Note: Not designed as a direct replacement for factory style blocks, some line modifications may be necessary. $119.99
^ what modifications do you think I would need for the Fiero? Would that wire plug in (or be wired in) right into the factory Fiero brake sensor? Does it still have all the factory safety systems in place then?
Bias is dirt simple. I'll first discuss "braking force ratio", rather than "bias", which may refer to the hydraulic pressure ratio. What's the convention?
My calculations are based on this information which I found on the forum; the numbers don't need to be exact, it's to get ballpark figures. Front static weight (lb) 1280 Rear static weight (lb) 1676 Wheelbase (in) 93.4 CG height (in)19.5
Because the CG is above the contact patches of the tires, this causes weight transfer on acceleration.
For best stopping distances, you want all tires to be equally on the verge of lockup.
With a stop approaching 0 g, the ideal F/R braking force ratio on a Fiero is 43F/57R. With a 1 g stop, the ideal F/R braking force ratio is 65F/35R.
These data points can be linearly interpolated/extrapolated to find the ideal braking force ratio at any acceleration.
With different caliper sizes, number of pistons, rotor size, etc...the braking force ratio can be set to whatever you choose. However, this will only be ideal for one particular acceleration.
Front wheel lockup will occur on a lower traction, lower acceleration stop. Rear wheel lockup will occur on a higher traction, higher acceleration stop.
The Grand AM upgrade at all 4 corners may actually be safer than stock brakes in the case of a proportioning valve failure. For low traction stops, stopping distance will be reduced.
Enter the proportioning valve. A device used to alter the bias based on acceleration! It's not psychic. Actually, it's a pressure sensitive device. Luckily for us brake tinkerers, hydraulic pressure and acceleration are directly related! Make sense? Press harder, stop faster?
A proportioning valve is a spring loaded piston with a preload connected to the rear brake circuit. Below the pressure required to overcome the spring preload, bias is determined solely by the brake hardware. Above the "cracking" pressure, bias is dependent on the spring rate of the spring, and the bore of the piston. Above the cracking pressure, the piston is depressed by the hydraulic pressure. This increases rear circuit fluid volume. When the mass of fluid remains constant, yet the volume increases, pressure goes down. This is how above the cracking pressure, rear circuit pressure rises more slowly than front circuit pressure, compared to pedal pressure.
A proportioning valve is not a perfect device. It does not make anywhere near an ideal straight line bias curve...rather it's kinked, and has a curve, and a straight line...
It's up to us enthusiasts to try and optimize this curve, to make it follow the ideal line as closely as possible, under most conditions.
The Pontiac Fiero is equipped with a proportioning valve, located in the combination block, end of story.
Fierobsessed, thank you for your response. I'm still somewhat confused though.
It looks like the Jegs part is what I need though. I just called them, and the sales person said that the electrical connector that the wires go to are to trigger the brake lights. Is that correct? Jegs phone # 1800-345-4545
Now he tells me I actually need this part: #555-63020 http://www.jegs.com/i/JEGS/555/63020/10002/-1 This looks like it'll give me the exact same problem as the HT-Motorsport Wilwood hack solution.
Could someone with more technical knowledge call them and confirm or deny this? I don't understand enough about brake bias and how this switch work to verify if the salesperson's explanation is correct.
Is that where the Fiero gets the brake light trigger from? Or does it get it from the pedal? The Fiero wire that goes into the OEM distribution block, does it send the "low pressure" signal to the idiot light?
Two things: 1) The e-brake is on, or 2) the brake pressure switch on the proportioning valve senses the brake fluid level is low and closes, completing a circuit, and turning the light on.
It's also supposed to turn on briefly in bulb test mode.
[This message has been edited by RWDPLZ (edited 10-05-2009).]
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10:07 PM
Oct 15th, 2009
Austrian Import Member
Posts: 3919 From: Monterey, CA Registered: Feb 2007
2) the brake pressure switch on the proportioning valve senses the brake fluid level is low and closes, completing a circuit, and turning the light on.
Does anybody know/think if the Jegs module replicates the factory function?
The brake pressure switch cannot detect a low fluid level. I have let the brakes run dry many times and can confirm this. The switch will only trip with a significant PRESSURE DIFFERENCE between the front and rear circuit.
I don't know which jegs module in particular you are looking at. I would recommend the simplest one with just the prop valve function.
Disable the stock valve - basically unscrew the prop valve cap, remove the spring and piston. Replace the piston without the spring, and screw back in. Bleed properly.
Install the aftermarket adjustable prop valve in the rear circuit. You can put this in the line between the master and the distribution block, avoiding cutting up the stock line.
Like this you will keep the brake light function.
[This message has been edited by pmbrunelle (edited 10-16-2009).]
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12:07 AM
Knucklebuster Member
Posts: 155 From: Chapel Hill, NC Registered: Sep 2009
If you've blead the brakes out: Then press the pedal really hard, The Combination valve will reset, and the brake light should turn off. I doubt its broken, but it will stick in that position till you do this.
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02:29 PM
Oct 18th, 2009
herb_mcracer Member
Posts: 30 From: McMinnville, Oregon. Registered: Nov 2008
Not trying to cause a conflict here, but fierobsessed says there is a delay function, but no proportioning in the combo unit. The Ogre seems to be saying the opposite, unless I mis-understood. (Trust me, I'm capable of misunderstanding anything, just ask my wife! ) By looking at the cut away, I can clearly see how the pressure balance sensing part of it works. And I think I can see how the proportioning part COULD work. (Smaller feedthroughs to the rears). But how does the delay to the front work?
------------------ You say my car smells like WD40? Don't they all?
Originally posted by herb_mcracer: But how does the delay to the front work?
It Doesn't....
Front disk brake delay is use with rear drum. Delay, Balance, and prop valve is car/trunk like old S10 Trunk.
Fiero has 4 wheel disc and not needed. Balance and proportioning valve only in Combination Valve in Fiero. Image I post from Fiero.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Originally posted by Fierobsessed:What are the Fiero caliper bore diameters anyway? I know the rears are smaller, as per normal.
The piston sizes, as measured on disassembled 1985 calipers, are:
Front: 48,95mm Rear: 47,6mm
If the calipers see equal pressure, this system is truly crap. There has to be a prop-valve somewhere in there to get a proper balance (the other option is to reduce the pressure in the whole system to avoid the rears locking up when driving in the wet, but that would result in extremely poor braking)
Keep in mind that a Ferrari 308 has 48mm front pistons and 38mm rear pistons and then a prop valve to reduce pressure for the rears. The Porsche 911 of approximately the same vintage use a similar setup.
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07:35 AM
Austrian Import Member
Posts: 3919 From: Monterey, CA Registered: Feb 2007
Does anybody know the model numbers, of the WIlwood calipers for Held Motorsports brakes? Did they get the correct Fiero ratio?
What is the difference between setting ration in the proportioning valve and in the caliper bore size? Also how does "crack point" come into effect in the proportioning valve?
Originally posted by PerKr: If the calipers see equal pressure, this system is truly crap. There has to be a prop-valve somewhere in there to get a proper balance (the other option is to reduce the pressure in the whole system to avoid the rears locking up when driving in the wet, but that would result in extremely poor braking)
No. See my post above. Post #p10 OE Prop valve does Nothing in Normal Brake. Brake System has the same pressure front and rear until it reaches 1000+ PSI (I think 1200+ for Fiero. I don't have a gauge that high...) Then close prop valve in rear circuit to stop pressure going any higher.
Basic Hydraulics cause balance front/rear in Fiero OE braking system until Prop valve is operated at 1000+ PSI. Is why rear piston is smaller that front. See cave. Brake Upgrade.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
The piston sizes, as measured on disassembled 1985 calipers, are:
Front: 48,95mm Rear: 47,6mm
If the calipers see equal pressure, this system is truly crap. There has to be a prop-valve somewhere in there to get a proper balance (the other option is to reduce the pressure in the whole system to avoid the rears locking up when driving in the wet, but that would result in extremely poor braking)
Keep in mind that a Ferrari 308 has 48mm front pistons and 38mm rear pistons and then a prop valve to reduce pressure for the rears. The Porsche 911 of approximately the same vintage use a similar setup.
How can you say this is crap? Do you know anything about the center of gravity of the Fiero in question? Or the 911 or 308 you mention? At least compare the Fiero to a mid-engined Porsche, not a 911.
Bias is not just determined by the piston diameter, but also the mean effective braking radius, and tires.
Let's not forget the proportioning valve.
For the record, there is a proportioning valve in a Fiero.
Originally posted by pmbrunelle: Bias is not just determined by the piston diameter, but also the mean effective braking radius, and tires.
Most car/trunk is True. But... In Fiero... Front and back brakes and tires are essentially identical. Tire size for some model, Like GT, are ignore. [GT tire size ignored? Look at Tire's Roll/Total Diam... use Miata.net] Same brake on all. Believe me. I've Check very carefully... Fiero Brake Bias is down to caliper piston bore. Fiero is weird that way.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Most car/trunk is True. But... In Fiero... Front and back brakes and tires are essentially identical. Tire size for some model, Like GT, are ignore. [GT tire size ignored? Look at Tire's Roll/Total Diam... use Miata.net] Same brake on all. Believe me. I've Check very carefully... Fiero Brake Bias is down to caliper piston bore. Fiero is weird that way.
Oh yes I agree. I meant about the 911 and 308.
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08:02 PM
Oct 20th, 2009
PerKr Member
Posts: 641 From: Mariestad, Sweden Registered: Nov 2006
How can you say this is crap? Do you know anything about the center of gravity of the Fiero in question? Or the 911 or 308 you mention? At least compare the Fiero to a mid-engined Porsche, not a 911.
Bias is not just determined by the piston diameter, but also the mean effective braking radius, and tires.
Let's not forget the proportioning valve.
For the record, there is a proportioning valve in a Fiero.
Weight distribution of the 308 is similar to that of the Fiero, so this is the one that should be closest in setup. The 911 has more of its weight over the rear wheels so should be able to use larger calipers on the rear compared to the fiero and the 308. The fact that it doesn't has to tell you something, which is why I brought it into the discussion.
The hydraulic/mechanical setup of the fiero results in a near 50/50. The Ferrari and the Porsche are (much) more front-biased to start with (despite the 308 having roughly the same weight distribution and the 911 having more rear weight bias). Are you telling me Pontiac got it right while Ferrari and Porsche messed up?
If it is true that the brake balance in the fiero is achieved by the piston ratios alone, then that would explain why my car has such poor braking: weight shift must not be allowed to happen. But I think the GM engineers were clever enough to figure that and actually add a prop-valve somewhere in the system (inside the combination valve), which is what I was trying to tell fierobsessed (who stated there is no prop-valve in the fiero).
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04:33 AM
Fierobsessed Member
Posts: 4782 From: Las Vegas, NV Registered: Dec 2001
I'll have to cut one up and take some component measurements. I'm still pretty sure there is no prop valve. Instead of arguing about it, I'll do some homework, and get back with some hard facts.
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07:16 AM
Boogaloo Member
Posts: 422 From: South Miami,Fl Registered: Sep 2009
The adjustable valve is designed for scratch built street rods and conversions using factory front brake lines. This valve is mounted in the front to rear brake line and controls only the pressure to the back brakes. The front brakes get full pressure and the rears are turned down until they do not lock up and provide a smooth stop. Adjustment depends on tire, caliper or rotor size and is different for all cars. Adjustable valves can also be used on factory or race cars to fine tune the rear brakes from locking up because of rear disc or mismatched components.
After you install it take it out on a not so busy road and adjust until it starts to lock up evenly or how you like it.
I'll have to cut one up and take some component measurements. I'm still pretty sure there is no prop valve. Instead of arguing about it, I'll do some homework, and get back with some hard facts.
There is a prop valve. Don't waste your time.
edit: please excuse this, the tone was a little rude
[This message has been edited by pmbrunelle (edited 10-20-2009).]
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04:48 PM
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
Originally posted by pmbrunelle: There is a prop valve. Don't waste your time.
there was a thread awhile back, where someone did in fact cut a cross section of the dist block. and - there was endless debate on whether or not that springed item was prop valve or not. still is no solid consensus. I dont think there is. but, I certainly would not bet on it. I cannot see any way that springed thing could in anyway limit hydraulic pressure. piston balancing is a perfectly acceptable way to balance a brake systems brake bias.
Originally posted by Pyrthian: I cannot see any way that springed thing could in anyway limit hydraulic pressure.
Lets try an analogy. Imagine you loosened the rear brake bleeders. Your front brakes would still grab, but the rears wouldn't. Agree the bias changed to nearly 100F, 0R?
As the brake pressure increases, the springed piston "gives" more. It's not as extreme as a completely open bleeder, but it has the same effect.
quote
Originally posted by Pyrthian: piston balancing is a perfectly acceptable way to balance a brake systems brake bias.
Piston balancing alone is not a perfectly acceptable method to set bias on a street car.
Optimal bias depends on the traction available. Piston balancing alone sets a fixed bias determined solely by the calipers.
Such a system optimized for dry weather would lock its fronts prematurely in the wet. Conversely, a system optimized wet weather would lock its rears prematurely in the dry.
A proportioning valve allows a variable bias ratio, dependent on brake pressure. The higher the brake pressure, the more the bias is shifted frontwards.
On low traction surfaces, brake pressure is lower, because the driver modulates brake pressure to avoid lockup, which occurs at lower brake pressure in a low traction situation.
On high traction surfaces, brake pressure is high because the driver is standing on the pedal.
Even with a proportioning valve though, you still need the proper caliper piston balancing to get you in the ballpark.
)
This looks like what I need.
By looking at the cut away, I can clearly see how the pressure balance sensing part of it works. And I think I can see how the proportioning part COULD work. (Smaller feedthroughs to the rears). But how does the delay to the front work? 
