I've heard there is a proportion valve to give the rear brakes less power then the fronts, like most/all cars.
I’ve also heard that is possible to remove it. Something about it being a spring or something.

Anyone care to explain the thing a little bit? Is it possible to remove it without replacing all the brake fluid?

TIA

It's also called a brake proportioning valve and all 4 brake line connect to it. It's made of a block of metal and has valves inside that allow for more fluid to the front than to the back because most of the braking power is needed in the front during a stop due to the transfer of weight during stopping. You can buy a universal valve that is adjustable which is most useful when an engine change has occurred. Good luck and peace.

Phil

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I am always available to help those that need help. Since my back injury, I cannot work on many of the aspects of the cars, but I can offer the knowledge of my 27 years as a GM technician. Bring the questions on................!

You can also affect brake bias by changing the size of the piston in the caliper. For example, if you install calipers with bigger pistons in the back, the brake bias will shift a little to the rear (or a little less to the front, depending on how you look at it). Personally, I prefer swapping calipers to messing with the spring in the combo valve. But that's just me.

Here is a link to a very good writeup of the proprotioning valve http://www.webelectricproducts.com/99/4/cvalve.htm

quote In the event of a brake failure, this spool shifts to the failed side. If, for instance, the rear brake line springs a leak, there won't be enough pressure build up in the line, and the pressure in the front circuit will shift the spool to the rear circuit side. When the spool shifts, it will raise the pin on the indicator switch, and that will light up your brake warning light (as if having the brake pedal go to the floor isn't warning enough).

Good stuff to know.

Alternately one could use a spacer to compress the stock spring more, but there is a limit to how much this can be done before bottoming out the spring which is something you don't want to do.

Aaron

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quote Originally posted by aaron88: Alternately one could use a spacer to compress the stock spring more, but there is a limit to how much this can be done before bottoming out the spring which is something you don't want to do. Aaron .

You're going the wrong way with the spring - lightening the spring will allow more pressure to reach the rear calipers

That’s not the way it works. In a hydraulic system the pressure everywhere is the same. The heavier the spring the more pressure there will be to the rear wheels. The proportioning system (not properly named in this case) works by moving the spring (only after the spring rate has been exceeded) to displace a portion of the fluid that would have gone into the rear calliper cylinders. A combination of the deflection and the spring rate directly controls the pressure.

Consider it a diaphragm that will move at a linearly increasing rate when the pressure gets too high. Therefore decreasing the output pressure but allowing to still climb.

The problem with the test done by the guy in Phil’s link is that he was testing on a corner. Testing the brakes on a corner is a bad way to do it because most of the tires ability to hold friction is already in use by redirecting the car. The tires will lock up much lower braking pressure in this situation and disproportionately as well since the car isn’t balanced properly for the tires (in most cases). There are so many factors involved here that this test should be done on a dry road with a person standing on the outside of the car to indicate when the tires are locking up. The proportioning valve in his case was set up to work best when in a corner. Most people don’t brake in the corner they brake before it, with the use of much larger forces.

Aaron

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Edit to make a correction:
Reducing the spring rate will increase the rear brake bias.

[This message has been edited by aaron88 (edited 01-18-2005).]

I believe if you study the cutaway view of the prop valve you will see that the rear brake plunger slows the rate of fluid flow via the tapered tip on the plunger. In doing so the rate of pressure build up in the rear calipers will be less than the front. If the spring pressure is increased on the plunger the tapered tip will not retract as quiclky ant the rear caliper will pressureize even slower. Yes if brake pedal is held down long enough the pressure in the front and rear calipers will be the same

It's not the rate of pressure build up... it's the pressure itself, like Aaron88 said. The volume taken up by the action of the prop valve reduces system pressue. However, I disagree with Aaron... I think going into a corner is a good way to do things. Ever heard of trail braking? As long as it's steady state cornering, there's really not much difference in fore/aft weight distribution vs. straight line driving.

What would throw it off would be if the turn were uphill or downhill.

quote Originally posted by Will: It's not the rate of pressure build up... it's the pressure itself, like Aaron88 said. The volume taken up by the action of the prop valve reduces system pressue. However, I disagree with Aaron... I think going into a corner is a good way to do things. Ever heard of trail braking? As long as it's steady state cornering, there's really not much difference in fore/aft weight distribution vs. straight line driving. What would throw it off would be if the turn were uphill or downhill.

The volume of the prop valve will not effect the overall pressure unless you reach the end of the stroke of the MC - that's the beauty of hydraulics

I may have misunderstood but in an earlier post I believe that the Ogre explained that the valve does nothing to alter the pressure to the rear wheels until the line pressure reaches a predetermined level, which seems to go along with Aaron88's posting.

Original Thread:
https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20041015-2-048392.html

Originally posted by Ogre:
"...The prop valve doesn't work like many people think it does. That valve only operates when you get way up on line pressure, like 600-700 PSIG. Under most driving you won't cycle the prop valve at all. You have to mash the pedal pretty hard for it to activate. Most of the time they won't trip in anything less than a complete panic stop.

An orifice in the valve will slightly delay rear brakes but doesn't affect total line pressure. That orifice is critical because you want the rears to always lag the fronts a tiny bit. Crap and corrosion buildup in the orifice or in the prop valve can increase the rear delay which would be a problem. The only sure fix is a new combination valve assembly. Trying to fix a crapped up combi valve is a bad idea..."

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RickN
White 88GT 5spd
White 85GT Auto

[This message has been edited by RickN (edited 12-03-2004).]

as said above, the pressure is constant, be it 4 psi, or 40 psi, its constant. its all volume. which is where I need help. I got the Grand Am brakes on the front, stocks in the back, and the larger volume of the GA piston is making my fronts work more, and I want to get more to the rear. so, from the sounds of what aaron88 was saying, I could shim up the spring. now the big question is - how much?

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[This message has been edited by Kohburn (edited 12-02-2004).]

Provided you have a constant supply pressure (and enough fluid) volume has nothing to do with the pressure supplied to either the front of rear caliper. If you have a 10 psi input to a container of 1 cubic inch or 100 cubic inches the outlet pressure of the container will still be 10 psi. If you could put a pressure meter on both the front and rear bake line and stepped on the pedal after a few moments the pressure in both would be equal. Piston area determines the force applied to the rotor . At 10 psi a 1 square inch piston will exert 10 lbs of force where a 2 square inch piston will exert 20 lbs of force, volume makes no difference. In the case of the prop valve as long as you have a pathway from the M/C to the caliper the pressure in the caliper will be the same as supplied by the M/C, it just may take longer to build up.

For my brake project, I was thinking of just chucking the proportioning valve and replacing it with two seperate 2-pound residual valves and an adjustable in-line pressure valve on the rear.

My other thought was to keep the stock proportioning valve but remove the rear residual spring and substitute an adjustable in-line pressure valve on the rear.

Any thoughts?

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Roy

double-click on this link to follow my interior build-up https://www.fiero.nl/forum/Forum1/HTML/023174.html

Blue 87 GT w/ 4th generation Firebird interior.
Suncoast Fieros

Judged "Best Custom Interior" at the 20th Anniversary Show @ Pontiac, Mich - 7/2003
Judged "Best Custom Interior" at the 8th Annual Fiero Fun Weekend @ Daytona - 3/2004

quote Originally posted by Phil: after a few moments the pressure in both would be equal.

right as designed it reduced rear brake pressure in the event of panic braking.. ie someone jamming on the brakes at the wrong time..

in the article the guy was basicly speeding up the flow to the rear allowing both front and back to lock up at same time

the only way to REALLY do brake proportioning right is either caliper piston diameter, brake rotor diamter, brake pad material (changing the coefficient of friction)

if i'm not mistaken no abs car uses a propotioning valve however most have smaller rear brakes than front

Hello,

I'm "the guy" that wrote that article. Here is what I found:

-If you increase the spring rate, either by putting in a stiffer spring or putting in a spacer, you will have less fluid pressure at the rear brakes, and hence, less braking force at the rear.

- If you decrease the spring rate, you will get more fluid pressure at the rears, and hence more rear braking.

When I finished my brake project, I had ultimately removed the spring. I found that to be the best balance for MY car, with MY tires, with MY new rotors, etc., etc. You must consider your entire car as a system and make modifications accordingly.

mrfixit88 is on the same track I was on, as far as the adjustable valve is concerned. When I pulled the spring out, I was planning to add an adjustabale valve in the rear circuit and then I could make easy adjustments without opening and bleeding the system every time I wanted to make a change.

But when I tried it without the spring, I liked the way it functioned, so I never installed the adj. valve.

If anyone has any other questions, I'll try to answer them, but I no longer have the car, so I can't run any other tests or take any specific measurements.

-L

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WebElectric Products

quote Originally posted by lawrence: Hello, I'm "the guy" that wrote that article. Here is what I found: If anyone has any other questions, I'll try to answer them, but I no longer have the car, so I can't run any other tests or take any specific measurements. -L

it was a very well written article

Hey Lawrence,

It's good to hear from you. Another thought I had (but was reluctant to post until you showed up) was about the ports in the P-valve. I noticed in the cut-away picture you have in your artical that the port for the rear brake circuit is much smaller than the port for the front (see the red circles in the picture below). I was thinking of drilling out the rear port larger to more closly match the size of the front port. This would allow more fluid volume to the rear calipers. It would also require the use of the seperate pressure for the rear brake circuit.

Roy

As far as pressure goes its all about volume of MC and Caliper. The MC feeds two ports into the P valve. But three lines come out. One each for the front wheels and one for the rears. The calipers are the same front and rear, but the front and rear get the same Volume/pressure? No. This would mean that the rear calipers each share the same volume as each front caliper and the volume and pressure is cut in half at the rear of the car. So the rear calipers get half the pressure as the fronts. That is if the front and rear pressure and volume are the same out of the MC. The lines on my 88's that come out of the MC are diferent size on the front (big) and rear(smal). This leads me to belive that the front to rear bias is not the same even before the P valve, at least the volume to the front is much more. Its more than the P valve that affects the front to rear bias.

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quote Originally posted by Rickady88GT: As far as pressure goes its all about volume of MC and Caliper. The MC feeds two ports into the P valve. But three lines come out. One each for the front wheels and one for the rears. The calipers are the same front and rear, but the front and rear get the same Volume/pressure? No. This would mean that the rear calipers each share the same volume as each front caliper and the volume and pressure is cut in half at the rear of the car. So the rear calipers get half the pressure as the fronts. That is if the front and rear pressure and volume are the same out of the MC. The lines on my 88's that come out of the MC are diferent size on the front (big) and rear(smal). This leads me to belive that the front to rear bias is not the same even before the P valve, at least the volume to the front is much more. Its more than the P valve that affects the front to rear bias.

I see your point about the duel front, single rear lines. But, to clearify, the real difference is volume, not pressure. Since the brake fluid is not compressable, once the pads are forced against the rotors (front and rear), the pressure is equalized throughout the system. But, because the rear circuit gets less volume, it takes longer to push the piston and force the pads aginst the rotors (thus, longer to equalize the pressure throughout the whole system). My though above was, if the rear port inside the proportioning valve was drilled out larger and the pressure spring removed, the volume of fluid to the rear brake circuit would be greater. Then, you could regulate the flow with an adjustable valve.

You never really want as much breake in the rear anyway. Too much rear brake will cause the back of the car to come around when you don't want it to.

[This message has been edited by mrfixit58 (edited 12-02-2004).]

ug...this is getting crazy. There is alot of truth in that post about it all being in the MC & the calipers. I think (i dont know) that in the MC, there is a different piston size for the front & the back brakes. which left me wondering why there would be a proportioning valve in the first place. then looking at what was going on, it seems the propotioning valve is not a proportioning valve at all, but more a delay for the rears, and has nothing to do with cutting pressure. but, I dont know. I just want a little more braking in the rear, after putting the Grand Am brakes in the front.....

mrfixit58, look at it like this. If the volume and pressure were the same out of the P valve. The rears would have half the brake pad pressure that the front will. Why? Because the piston size of the rear calipers combined is twice that of the front. Each front gets the same volume as the two rears combined. That is the same as having calipers with twice the size pistons. If you dont send more volume to the rears they cant make the same brake pad pressure. The larger piston would make more brake pad pressure if there were a higher volume to move the piston farther. But if they get the same volume they move half the distance and wont have nearly the same brake pad pressure.

Now I realize that the front and rear volume out of the MC may be diferent in our Fieros but if all things were the same the front two calpiers will have more clamping pressure then the rear, because the rear will each get half the volume than the each front.
Did I explane it OK or do you want me to try again?

The hydrolic system is complex, that is why one system that works great on a Ferrari will not work on a Fiero. They are "tuned" for our cars from the factory. If a person wants to modify the brake system, i sure hope they do there home work first.

As I understand it, the purpose of the prop valve is to control the flow rate (i.e. volume) of fluid going to the rear calipers. The purpose of the spring is to control the fluid flow, based on the pressure coming out of the master cylinder. A stiffer spring will require you to push the brake pedal harder to generate fluid flow to the rear calipers, and vice versa.

The basic principle is that when you make a panic stop (i.e. you're pushing the brake pedal very hard), the prop valve opens up, allowing more fluid to flow to the rear calipers. The idea here is to help prevent front brake lock-up during panic stops. Unfortunately, the prop valve is set so conservatively that it doesn't perform this function very well (for fear of stupid drivers spinning out in turns).

I can understand switching to a softer spring in the prop valve to shift the brake bias to the rear. But I think completely removing the spring is a bad idea. Removing the spring deletes the braking system's ability to adapt to different braking conditions. I would not recommend doing this to a car that's driven on public roads on a regular basis.

The size disparity between the front and rear caliper pistons doesn't help the situation, either. In the stock braking system, the rear calipers have smaller pistons than the fronts. This results in the rear brakes having less clamping force than the fronts at all times. In my opinion, this is bad because it reduces overall stopping force and puts more burden on the front brakes (i.e. greater chance of lockup, and more wear). It also means that if your front brakes lockup, you're basically SOL.

[This message has been edited by Blacktree (edited 12-02-2004).]

quote Originally posted by Rickady88GT: mrfixit58, look at it like this. If the volume and pressure were the same out of the P valve. The rears would have half the brake pad pressure that the front will. Why? Because the piston size of the rear calipers combined is twice that of the front.

I understand what your saying but I wasn't talking about pad pressure. I was talking about line pressure. Two square inches of pad at 100 psi has twice the clamping force of one square in at the same pressure - true. My discussion is simply looking for a way to increase the volume to the rear brake circuit to allow me to adjust how much bias I prefer for my brake upgrade.

Please refer to the picture of the P-valve and you'll see a balance switch at the top. The horizontal shaft below the electrical terminal stays in the middle as long as the pressure is the same front to rear. If a pressure drop occures, the shaft slides to one side completing the electrical circuit for your brake light. While more fluid moves through the front of the P-valve, the pressure remains the same as the rear.

Roy

OK I see. In that case I woud get five or six P valves from a junk yard (not hard to find around here, nobody ever seems to take them) and have that small hole redrilled in each P valve. Make the hole a little larger in each P valve. And try each one. See what will work best. I think the spring would be the first place to start thou. Modify or make a new end cap/bolt that holds the spring in so you can use a lighter spring, then add shims to the spring tell you get the results you want. That is if you want to stay with the stock Fiero parts. It would be easier to get an adjustable after market P valve.

Would prolly be easier to devise threaded adjustment, rather than use shims...

There seems to be a general lack of understanding of how a prop valve works...

When the driver pushes on the brakes, the master cylinder piston moves and pushes fluid into the brake system. As the front brake pads contact the rotor surface, the MC piston doesn't move much more. The pressure in the front circuit is determined by the force the driver's foot is exerting on the system.
Meanwhile, in the rear circuit, pressure is determined by a combination of the force on the MC piston and the force exerted by the prop valve piston. As the driver pushes harder, the increase in pressure pushes the valve back further, but because of the spring on the valve, the line pressure rises more slowly relative to pedal force than does the line pressure in the front brakes.

That's the fundamental effect of the proportioning valve. Once the pads meet the rotor and the driver pushes harder, the line pressure in the rear circuit rises more slowly relative to force on the pedal than does line pressure in the front circuit.

A treaded type of adjustment would be easier to adjust as you wont need to bleed the brakes every time you change a shim. but a treaded adjustable bolt would have the potential of leaking. The fluid woud seep out but that is not a big deal. The problem is air seeping in. Again it might not be a big deal but the potential is there. For an adjustable bolt I would use a type of diaphram like a fuel pressure reguator. But that is a major mod.

quote Originally posted by Rickady88GT: As far as pressure goes its all about volume of MC and Caliper. The MC feeds two ports into the P valve. But three lines come out. One each for the front wheels and one for the rears. The calipers are the same front and rear, but the front and rear get the same Volume/pressure? No. This would mean that the rear calipers each share the same volume as each front caliper and the volume and pressure is cut in half at the rear of the car. So the rear calipers get half the pressure as the fronts. That is if the front and rear pressure and volume are the same out of the MC. The lines on my 88's that come out of the MC are diferent size on the front (big) and rear(smal). This leads me to belive that the front to rear bias is not the same even before the P valve, at least the volume to the front is much more. Its more than the P valve that affects the front to rear bias.

I don't think the volume matters. It's the cross section of the M/C and caliper bores that matter. Remember, it's Load per Unit Area, and the Unit Area is the surface of the piston. Indirectly, this does correlate to volume, but it's the pressure on the piston that governs the force.

The reason there are three outlets is so that there are three separate circuits in the system. This might even be law. Again, it doesn't matter how many lines or even how large in diameter the lines are, it's the displacement that counts. The lines are full. The only displacement is to make the pistons move, and that's not much.

-L

This is a cool subject. But I have to say that I dont fully understand what you just said lawrence. can you elaborate?
I did not look at the exact size of MC or the P valve but I just explained the basic idia of the Fiero brake system. I have a WCF 12" C4 converson. So this is of intrest to me. I do want to get the most out of my braking system as I can. So if you can explain more clearly I would like that.

quote Originally posted by lawrence: I doubt that drilling the holes will do anything. There is such a small amount of fluid that actually moves that hole size probably doesn't come into play.

Here's where I beg to differ... The volume of fluid moving though the system is what detrmines how quickly the pads reach the rotor. Once the pads come in contact with the rotors, the pressure begins to rise. Let's call this the jam point. Once you've reached the jam point, this is, as you pointed out, where there is very little fluid moving and, I believe, the internal spring in the P-valve no longer serves a purpose. The harder you push the peddle, the more the internal pressure will rise.

Larger volume calipers will require more fluid before they reach their jam point. If you use larger calipers and the stock master cylinder, you may notice more peddle travel but less peddle pressure (on your foot). The reduced peddle pressure is due mostly to the increased the clamping force at the brakes as a resut of a combination of larger calipers (more clamping area) and a larger diameter rotor (better leverage). To most people, this will feel amost spongy. But, as long as there is enough fluid moving through the system to lock up the brakes, everything works well. To combat that spongy feeling, most will replace the stock master cylinder with one that has a larger bore. This will increase the peddle pressure because you will move more fluid with less peddle travel. I believe that there is a good technical article in Ogre's Cave that explains this.

Interesting discussion.

Roy

quote Originally posted by Rickady88GT: This is a cool subject. But I have to say that I dont fully understand what you just said lawrence. can you elaborate? I did not look at the exact size of MC or the P valve but I just explained the basic idia of the Fiero brake system. I have a WCF 12" C4 converson. So this is of intrest to me. I do want to get the most out of my braking system as I can. So if you can explain more clearly I would like that.

as an 88 you are only increasing the rotor diamter which will reduce the amount of pressure at the calipers required to slow the car..
it will not affect pedal travel - for that setup I'd say the most you'd want to do is put a weaker spring in the valve to make the rear brakes react a little faster

quote Originally posted by Rickady88GT: This is a cool subject. But I have to say that I dont fully understand what you just said lawrence. can you elaborate? I did not look at the exact size of MC or the P valve but I just explained the basic idia of the Fiero brake system. I have a WCF 12" C4 converson. So this is of intrest to me. I do want to get the most out of my braking system as I can. So if you can explain more clearly I would like that.

Which part?

When you press the brake pedal, you pressurize the system. That Pressure (pounds per square inch) is applied to the surface of the caliper cylinders of Area A-squared. Multiply the Pressure by the Area and you get pounds (force). This is how much lateral force is being applied to the pads.

Not dissing anyone, but I believe everyone is over-thinking the proportioning valve spring. It is a pressure regulator, and that's it. There is no timing or delay involved, other than that caused by the physics of the parts, and this is not part of the design. If possible, a design with massless parts would be used so that there would be no delay at all.

quote Originally posted by mrfixit58: Here's where I beg to differ... The volume of fluid moving though the system is what detrmines how quickly the pads reach the rotor. Once the pads come in contact with the rotors, the pressure begins to rise. Let's call this the jam point. Once you've reached the jam point, this is, as you pointed out, where there is very little fluid moving and, I believe, the internal spring in the P-valve no longer serves a purpose. The harder you push the peddle, the more the internal pressure will rise. Larger volume calipers will require more fluid before they reach their jam point. If you use larger calipers and the stock master cylinder, you may notice more peddle travel but less peddle pressure (on your foot). The reduced peddle pressure is due mostly to the increased the clamping force at the brakes as a resut of a combination of larger calipers (more clamping area) and a larger diameter rotor (better leverage). To most people, this will feel amost spongy. But, as long as there is enough fluid moving through the system to lock up the brakes, everything works well. To combat that spongy feeling, most will replace the stock master cylinder with one that has a larger bore. This will increase the peddle pressure because you will move more fluid with less peddle travel.

Nope. It's pressure. The diameter of the M/C translates your size 11's into PSI. As it gets larger, you will get less PSI. On the caliper end, if the diameter of the cylinder gets larger, you will get more force applied to the pads.

If you were to design a perfectly rigid system, with absolutely true rotors and bearings, the pads would ride almost up against the rotors. In this case, the only thing pulling the pads away would be the oring that pulls the cylinder back into the bore. Now let's say that we have magic orings that wouldn't do that.

So, now, there will be NO movement of the pads when you apply the brakes. Right? Do you think the brake system will work? There will be "no" fluid moving, so no delays, no small passages to worry about, etc. Yes, it would work WELL.

I'm an M.E., but I don't have any special training in brake system. Does anyone know a Bendix brake engineer we could talk to?

-L

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WebElectric Products

lawrence You said "I dont think the volume matters. It's cross section of the MC.........."
This is where I disagree with you, or maybe I just misunderstand you. Because in the system as it is set up, volume is a huge deal. The rears get less than what is sent to the front. I do agree with you that the P valve is a pressure regulator. As this is how the shop manual describs it. But if the rears got the same volume as the fronts the fluid would have to travle MUCH faster than the fluid that goes to the fronts. The fronts have two lines to supply the calipers and the rears have just one. And the rears may also have a smaller piston. The fluid to the rears is not just pressure regulated it is also volume regulated via that little passage in the P valve. Volume does matter because the volume moves the pistons and pressure clamps the pads. You need both to work the brake system. It is a delicate balance of many components to get the car to brake right.
My point is this,.... to get better braking in the rear you must look at all the components to make the right choice of mods. What works for me may not work for everybody.

http://auto.howstuffworks.com/disc-brake.htm

Don't intermingle drum brake thinking with disc brakes! While a larger disc brake caliper piston does hold more volume, the braking system really doesn't see this when the brakes are applied. What the brake system sees is pressure being applied to a volume that is already in place. Disc brake fluid is virtually a stagnant pool of liquid. The master cylinder is not a pump as much as it is a pressurizer.