Brake Pressure Question

I've been working on my Dart getting some things done so it will be ready for warm weather. It had a bad master cylinder that Cass took care of me on with a replacement. The new one is just like the one I removed, 15/16" bore. The car is a manual brake car with factory pedal, Wilwood disks on the rear and Strange on the front, adjustable proportioning valve and line lock in the front line. It also has a gutted 73 distribution block that only the front lines go through.

I've had a 1 1/32" mc on it before and the pedal was like a rock, the 15/16" was better but it still doesn't stop like it did with the 73 disk/Wilwood iron mc set up.

Last week I bought a Strange pressure gauge and adapter to check the pressure. So after bleeding the brakes I checked them today. Strange suggested putting pressure on the pedal like it was a surprise stop so that's what I did. The rears had 800# and the fronts were normally 350-400 although one time one side did go to 500#. I double checked the adjustable proportioning valve to make sure it was wide open.

Is that normal?

BTW Strange recommended 15/16"- 1" on the mc.

That sounds low to me.

I like 1200.

But I would verify that. I sold my race car a decade ago.

Almost sounds like you have the front lines hooked to what should be the rear outlet at the master.

The fronts are on the rear port (closest to firewall) on the mc.

It sounds like the master is plumbed backwards or something is mis-matched. On the Strange Mopar style unit, the port nearest the pushrod supplies the highest volume of fluid. Typically on a drag car with big tires and larger brakes on the rear, that is where the rears are plumbed to. I would look at the Wilwood calipers and compare the bore size to the Strange units. I've had certain Wilwood calipers with small pistons relative to what Strange and others use. I've had to go to a 7/8 master to make the brakes work with those. I stopped using them, but still have to deal with them sometimes. I also have not needed or used a prop valve in years, once the system is set up correctly.

The Wilwoods are 1.38" and the Strange have 1.625 and 1.750" pistons.

Thanks

If the proportioning valve is doing nothing, and the shuttle valve is doing nothing, then all pressure front and back would have to be equal.

Because all of the oil is coming from a common same size bore I don't know how you would get those different pressure readings.

As such, one of those valves must be doing something.

My old adjustable prop valve would still reduce pressure even at the "least" setting. I would take it out and put a coupling there to start.

Get rid of the prop. valve. If this is a drag car the rear should have more braking then the front and vice-versa for the street. Manual brakes are just that. The harder you push, the more pressure you create. Sometimes it takes a pump or two to build high pressure. I always really liked manual brakes for their feel and both of my cars have them.

Thanks guys. I'll check today to see if anyone has a coupling. This a street/strip car.

The rear brakes don't go through it.

I thought the wildwood prop valve produce more pressure when turned in. Just the opposite you would think. Try that

It's not a Wilwood valve but I did adjust it and check to make sure it was wide open.


I just got off the phone with Wilwood, they said it's not far off on the rear pressure but the front is low. The tech suggested I start at the mc checking pressures and work my way through the system. I'm going to remove the adjustable proportioning first and check the pressure. If that doesn't improve it I'll fab up lines and go through the system.

Thanks guys, Justin.

Do the math; with a 1 1/32" MC, 6-1 pedal ratio and 100 lbs. leg pressure your line pressure should be 716 psi.

http://www.wallaceracing.com/brake-master-cyl-calc1.php

In your case, the unknown factors are pedal ratio and leg pressure. "Surprise stop" leg pressure will vary with the individual, bathroom scales between your foot and the pedal will give you a number to compute from.

There you go. Do the math to figure the difference in piston area x eight pistons. The bigger the piston, the more pressure is applied to the pads.

Put a decent caliper on it, plumb the rears to the MC port closest to the pushrod if the rear tires are larger than the front, throw the prop valve out. Expect to need the MC bore to be bigger with appropriate calipers on there.

On your A-body the line pressure should be in
the 1700 to 2000 psi on MAX breaking.. it does
sound like the lines are backwards

to get the most accurate pressure reading, the gage should be at the caliper bleeder, correct ?

That's where I took mine.

Consider 500psi of fluid pressure at one 4-piston caliper.

With 1.38 pistons, the force exerted on the pads is 2960lb. total.
With 1.75 pistons, the force exerted on the pads is 4800lb. total.

Or another way to look at it - I can get just about the same amount of force on the pads with 4 1.75 pistons @ 300psi as I can with 4 1.38 pistons @ 500psi.

The calipers are not correct for a skinny front tire-big rear tire application. The front brakes are being forced to do all the work. Knowing the fluid pressure is useless without taking caliper piston bore size into consideration. If you have the same caliper on all 4 corners of the car, then you can compare front and rear pressures and get an idea of what's going on.

A lot of Stockers and Super Stockers have gone to four 4-piston calipers on the rear with either two 2-piston or two 4-piston calipers on the front. All with the bigger pistons. Then matching MC size to the total fluid requirements of the system.

You have a problem somewhere in your front system for the pressure to be low like that. Check the pressure at the MC if you can. Or check it at some other points in the system if you can get access. You might have a crimped line or some blockage in the system.

Andy, I have the fitting to go in the line to remove the adjustable proportioning valve and will most likely get it changed today. I'll go over the lines to see if there are any kinks, it has 3AN flex lines. When I get some help I'll check the pressure again.

CMcAllister, the fronts are Strange race brakes I bought from Hemi Fred years ago, the only other option I see for calipers on these brakes are 1.75" pistons. I can go to a larger piston on the rear Wilwood's if needed but their tech didn't think I needed to. Once I get the pressure figured out I'll see how it stops and make any needed changes.

Thanks guys, this brake stuff is greek to me. lol

Even if a line is kinked but not totally closed
off that pressure will finally equal out on that
line..in a period of time.. same thing as a prop
valve... this just hyd 101

Understood, the proportioning works an adjustable orifice restricting flow, thus initially pressure too. In a closed system like brakes the pressure will eventually equalize on both sides of the orifice.

Right now I just want to find why my pressure isn't where it needs to be on the initial pedal pump.

The proportioning valve is out and the union is in. Have to work tonight so hopefully tomorrow I'll re bleed the fronts and see what I have.

Thanks, Justin.

It would seem that way but it isn't; when computing the clamping force of multi-piston rigid calipers you only use half the caliper pistons. IOW, only one side applies force, the other side acts as a backup. I know this is counterintuitive but that's the way it is.

After re-reading the tech information I was using, I believe that is correct. I had just skimmed through it earlier. The numbers can be divided in half. But the percentage of difference between using the 1.38, instead of the typical larger bore units, is still valid.

thanks !

I don't think I would want more stopping on the back, street or strip. Once it starts sliding the car tends to swap ends. Did you bleed that MC good? I would start the guage at the MC and work down.

On a drag car with big rear tires you want more
breaking in the rear.. if you have more breaking
in the front it will try to swap ends.. a street
car the breaking is 70/30 most on the front.. but
the tires need to be matched or close.. the tires
do the work..also if you run skinny fronts they
will try to lock up and skid(possible blow outs)

Without knowing weight bias, you can’t really say it should be either way. I can tell you that my own car, that had countless hours of experimenting on the brake system, was as well balanced (brake wise) as any car I had ever driven. It was a mish mash of odd parts that worked in orchestra on a heavy car that had no business going as fast, or stopping as quick as it did. Each front wheel (26” tall Eagle front runner) had 1100 pounds static weight, while the rear (31” 10.5W MT) were at 750 pounds static weight.
The car had 12”dia vented front rotors with wilwood large piston calipers. The rear of the car had 10” drum brakes from a cuda or some such car. The master cylinder was what you would find in a late 80s caravan. Those master cylinders have no built in bias because they are designed for a cross braking application. My adjustable prop valve was in the rear brake circuit. I adjusted out some rear brake until i could skid the car evenly at speed.

On my P-body I had front runners and 31x14 the
adj prop was on the front.. master was a 3/4
mopar unit.. I could lock all 4 tires if I had to
but only did that testing the prop till it was set
right(disc on all four)

I was under the impression that the aluminum master cylinders that are popular on our cars don't have any bias built into them, but they do have two different volumes for each side of the reservoir which is why the rear port is supposed to go to the larger volume system on the car.

Brakes are bled again, it's pretty easy to get 900-1000# on the front now. I could only do the drivers side as there is no way for me to see the passengers side.

Not all alum master are biased.. my 3/4" master isnt but
a lot of larger masters are.. you would need to check
what you have.. mine has the larger side also.. I have
a couple of alum masters that are the same size.. I got
a FEW masters when I was working in the brake lab( I was
on loan to the brake lab but my room mate worked there
all the time)

I have never seen an aftermarket proportioning valve that worked or functioned this way. Every adjustable prop valve I’ve ever come across is an accumulator/spring that can only reduce pressure... not flow. This is why opening / unscrewing the valve decreases pressure (less spring load absorbs more fluid into the small accumulator. I guess technically flow is slightly reduced by the fact that a vey small amount of fluid is absorbed in the accumulator, but there is no flow orfice.
Also, pressure will not equalize using any of the adjustable proportioning valves I’ve used. Can anyone show me one of these flow style valves? Maybe I’m missing something.

When you say a master cylinder is biased, are you saying you get different pressure out of one line VS another? Is that possible?

I don't think it is.

When a car is designed, the size of pistons front or back gives the manufacturer the calculated brake capabilities.

And that factory valve that most call a proportioning valve is not that at all. It is a shuttle valve that gives you the dual/failure mode braking if front or back leak/fail. This is not done in the master cylinder.

I've been wrong before, so I am correctable, eh?

Finally someone got it right, thank you. unbelievable the amount of wrong info.

Yes you can get different pressures out of the master cylinder. I understand your query. Since the bore is the same, it would seem that output would be the same. The way you can get a pressure bias is through piston surface area. The rear most piston has a centered semi floating rod that deducts from the overall surface area of that piston. The second piston has no rod, or less of a rod. More surface area= less pressure.

If the bore is the same for both piston cups there will be no difference. A rod pushes the rear cup and fluid pressure alone pushes the from cup to make up for clearance issues. Has anyone ever taken one of these apart to see how they work?! Or is this something you read off the net.

There are step bore master cylinders also, they have different pressure & volume on the ports.

Prop valve

What does a proportioning valve do?

A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions.

The set up above is for a car with equal sized tires

The OD of two pistons can be the same, while still having different surface areas. Being a fat guy, I will use doughnuts as an example. There aint as much doughnut there when the center is gone.

Think of a hydraulic cylinder. The rod side has less surface area to apply power to the load.

Well, this is interesting.

Let me approach differently. Why would the factory want a different pressure front to back when they can change brake "bias" by changing piston area, rotor diameter, drum diameter, width.

To have different pressure the pistons would have to be hard connected to each other. Maybe some are.

If in the master, the front and back reservoirs and pistons were totally independent of each other, the pedal would stop as soon as say, the rear flow stopped. Once hard in the rear, no flow would go to the front, hence no brakes up there.

Isn't it interesting that the brake system, is far more complex than meets the eye, eh?

Please correct me if I am wrong, I think a lot can be learned here.

Because most trucks have more rear brake than they need when not loaded(talking drum and wheel cyl size)and even with down stream regulation (ride height, rear abs dump etc) rear volume to apply brake shoes can be higher than front discs, while rear pressure can be lower.
As far as m cylinder pistons being connected... that happens through hydraulic pressure.
Another form of bias inside the master can be controlled through the transfer springs. The transfer springs can be installed to which chamber compresses first.

there is a spring between the 2 bores that will
bias the master(on most cars now days)

The old minivans had a prop valve mounted at the rear, attached to the rear axle with a link. As the rear suspension was loaded heavier or lighter, it would adjust the valve which would increase or decrease rear brake bias.

Once you modify your car from OE, bigger or smaller tire, different brake kit, lighter weight or substantially different weight bias, etc., the OE calibrations go out the window. With any major modification and/or replumbing, the OE prop valves, distribution/compensation blocks, etc., hit the can. I don't care if they are a dime a dozen, I won't use an auto part store master if I don't know all about it. Even an oval track and a drag car system and hardware are completely different. Drag race cars need parts designed and configured for drag racing. Just like the parts that make up an engine - if the brake system is a collection of stuff from different manufacturers, picked out of a catalog based on price or bad information, mis-matched, sized wrong, being used for the wrong application, or OE crap being used for something it wasn't intended, it won't work right.

"the front and back reservoirs and pistons were totally independent of each other, the pedal would stop as soon as say, the rear flow stopped. Once hard in the rear, no flow would go to the front, hence no brakes up there."
The reason the rear port goes to the front brakes, they do the majority of the braking, and why discs where used up front. Any decrese of pressure its safer at the back. Everybody knows In the extreme panic stop, a car nose dives, effectively raising the rear almost to the point of no tire contact, with equal pressure, those brakes will lock. This happens with all tires the same size and big fat slicks, tires unload. Granted, center of balance has a lot to do with how the brakes are set up on each type of car. Low light car can be set equal to an extent, normal B-body, you better take some out of the rears and have more up front.

Spring is just used to keep the two pistons separated so brake fluid fills the void and acts as a "liquid" rod, so they move in unison.

Have you ever built a chassis car.. the rear brakes
do most of the work due to size of the rear
tires.. on a chassis car has almost zero dive on
the front

I think we all agree that once we depart from factory specs, brake bias needs to be considered.

I guess I'm just saying I still don't believe you get two pressures out of the same master. From the factory, there would never be an instance that this would be needed.

Now, I understand differential in pressure and how that is obtained. I was pretty good at working on the Navy's Mk 45 gun system. It has some of the most sophisticated hydraulics ever. I get that. Actually, I taught a lot of that.

That mini van adjustment, never knew that. But I did rent many an early 80's Citation front driver GM that had the rear brakes set up for 4 people in the car. When you only had one or two people, it wanted to swap ends.

I will post a pic of a typical pre abs combination valve. Im doing this from an iphone so may not work😀
Most cars built from say ‘72 and later had valves with these features. First, yes, a shuttle valve to lock off a circuit failure, and indicate to the driver that there is a failure (warning light)
Next, the rear proportioning accumulator section.
Also note the front circuit has a spring loaded restrictor. This feature assures the rear circuit starts to react first. Mostly to take up slack in the rear shoe adjustment without shifting the shuttle section of the valve.


It is termed a combination valve because it has 3 functions. To say it is not a proportioning valve at all is a stretch.

As for not believing that a master cylinder can output 2 different pressures, that’s fine with me😂

That is a great picture. Thanks for posting. Learned something today.

As the pic shows, less pressure to the back. The propositioning part came in after disc showed up with their better stoping problems. Now it's anti-lock to keep the wheels turning instead of sliding. Traction control is another variant.
And yes I've built a chassis car and I wanted both close with a tad more bias to the front. Most any brakes are fine till you lock them, then you find out to late how well they work.

I've never had the brakes lock up after I got them adjusted
but you can do what ever you want

Unless they are heavy (2600+), most of the tube cars I've seen or dealt with have two 4-1.75" piston calipers on the rear and two 2-piston or even single piston calipers on the front. Basically just enough brake to be able to do a burnout. With 14" tires on the rear and 3.5 or 4" on the front, having too much front brake is asking for trouble. A lot of Super Stockers and even Stockers are going to 4 calipers on the rear so they can use 4 piston calipers on the front and race the stripe and/or get the car stopped without sliding the front tires.

Something to add to the mix here.
So on a street/strip car with a 2x3 tube chassis with 14 point cage running 7.5" x26" tire up front and 33" x 19.5" or 22" rears, 4 wheel Wilwood discs. What would be the optimum bias setting?