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.