1972 Z Brake switch / valve

[Patcon]

When you say you have a hard pedal, does this mean you are driving the car and the pedal pressure is very high? or are you in the shop and the pedal doesn't travel and feels overly hard?

What master did they run with the MK63 setup?

With the car on jackstands and you press the pedal does it lock the wheels?

[zKars]

A hard pedal where you get immediate excessive braking at very distinct pedal point, has historically been commonly caused by the loss of the "reaction disk" in the booster. It's amazing how this thin little disk of rubber gives you the soft pedal and modulation control that utterly lost without it.

[JLPurcell]

2 hours ago, Patcon said:

When you say you have a hard pedal, does this mean you are driving the car and the pedal pressure is very high? or are you in the shop and the pedal doesn't travel and feels overly hard?

What master did they run with the MK63 setup?

With the car on jackstands and you press the pedal does it lock the wheels?

The pedal is hard with very little braking force. I currently have the car on jack stands and will get someone to help me test which wheels are locking etc...In the past I ran a MK63 non-vented rotor set up on the stock master cylinder.

[Zed Head]

5 hours ago, JLPurcell said:

A few months ago I found a set of NOS MK63 calipers (for vented rotors). They had been sitting for years. I found rebuild kits took them apart had them cleaned out and reassembled them. I found a set of vented rotors and installed the upgrade.

The system worked well prior to the upgrade.

Since installation I have a hard pedal.

When I pump up the brakes and then start the car. the pedal moves just slightly.

When I removed the hose to the booster is was holding vacuum

I am ready to put the stock system back in place and drive the car! 

and.....  "The pedal is hard with very little braking force."

and.... "help me test which wheels are locking etc"

 

I pulled out the information as I found it.  It seems like you're saying that you only swapped front calipers and rotors and left everything else exactly the same.  Is that right?  You described a lot of stuff that you did before, but the brakes apparently worked fine with those things done.  So they kind of don't matter.  What would the "stock system" be?  More than just rotors and calipers?

You said hard pedal, and pedal is hard with little braking force.  But then you talked about locking wheels.  That would be very much braking force.  

And you mention pumping up the brakes before you start the car, but not while the engine is running.  Do the brakes pump up with the engine running?  The pedal gets higher and harder with pumping?  That's a sign of air in the system.

You didn't say which end of the hose you removed, and how you know it was holding vacuum.

Sorry to pick apart your writing.  Can't tell what you're trying to describe though.  It's not clear if the brakes even work at all, in slowing the car.  It kind of sounds like the caliper pistons are locked up in their bores.  No fluid is flowing, so the pedal does not drop.  But it can flow when the bleed valves are open.  Fluid does flow when bleeding the calipers right?  The bleeding is a test of the hydraulic system.  You could remove the calipers and have somebody gently press the brake pedal to see if the calipers move.  Put a thin board between to be safe, thinner than the rotors.

[240260280]

What is the diameter of the master brake cylinder?

How many pistons are in each brake and what are their diameters?

 

There are some brake systems designed for no master-vac (depending on the ratios of pushing piston to pushed pistons) so if one it used then you will get instant braking.

[JLPurcell]

7 hours ago, Zed Head said:

and.....  "The pedal is hard with very little braking force."

and.... "help me test which wheels are locking etc"

 

I pulled out the information as I found it.  It seems like you're saying that you only swapped front calipers and rotors and left everything else exactly the same.  Is that right?  You described a lot of stuff that you did before, but the brakes apparently worked fine with those things done.  So they kind of don't matter.  What would the "stock system" be?  More than just rotors and calipers?

You said hard pedal, and pedal is hard with little braking force.  But then you talked about locking wheels.  That would be very much braking force.  

And you mention pumping up the brakes before you start the car, but not while the engine is running.  Do the brakes pump up with the engine running?  The pedal gets higher and harder with pumping?  That's a sign of air in the system.

You didn't say which end of the hose you removed, and how you know it was holding vacuum.

Sorry to pick apart your writing.  Can't tell what you're trying to describe though.  It's not clear if the brakes even work at all, in slowing the car.  It kind of sounds like the caliper pistons are locked up in their bores.  No fluid is flowing, so the pedal does not drop.  But it can flow when the bleed valves are open.  Fluid does flow when bleeding the calipers right?  The bleeding is a test of the hydraulic system.  You could remove the calipers and have somebody gently press the brake pedal to see if the calipers move.  Put a thin board between to be safe, thinner than the rotors.

The brake pistons are 1.63" each total of 8. The master cylinder is the stock 7/8". The stopping force is limited but the vehicle will stop when the brakes are applied with hard pedal force. The pedal pumps up almost immediately and is hard with no give. When you pump up the brakes, engine off, and then start the engine you can feel a slight give in the pedal. The booster hose removed on the master vac side of the one way valve is holding vacuum. I hope this is a better explanation. My explanation has gotten mixed in with answering other queries. thanks

[240260280]

Your calipers have more piston area than stock thus the longer stroke required.

A 15/16th master will bring the stroke to less the 10% of stock stroke to achieve the same braking force.

 

If the reaction disk has fallen then your required stroke range will be compromised.

Edited April 24, 2018 by 240260280

[Zed Head]

5 hours ago, JLPurcell said:

The brake pistons are 1.63" each total of 8. The master cylinder is the stock 7/8". The stopping force is limited but the vehicle will stop when the brakes are applied with hard pedal force.

So, basically, part of the hardness issue is about how hard you have to press the pedal to get the car to stop.  You're saying that the brakes don't work better, they work worse.

It sounds like the booster works correctly.  It's hard to tell how far the pedal is moving when you're pumping the system up, and how many times you pump.  That would be a clue about whether it's air in the system or something else.  Pumping the brakes is just a way to compress air bubbles.  The fluid doesn't get pushed back out of the caliper faster than your foot can push more fluid in to it.  So, if th pedal moves a long distance and it takes a few mups, that's a big air bubble.  

On the other hand, if the fronts don't feel like they're working after being pumped up, then you might have a pad and/or rotor problem.  Any chance there's something slippery on them?

 

Once the system gets pumped up and the air bubbles are compressed then it should behave like it will normally.  If the pad friction on the rotors is sufficient you'll get braking force.

Hope that helps.  You need to separate the system in to its parts and think about each.  Here's a clue about big bubbles in the calipers - the brake check light should come on if your switch is installed and working.  Mine did when I had my stock calipers installed upside down.  It was a clue that made more sense when somebody noticed a very large quantity of fluid leaving the reservoir and coming back as I worked the brakes..

[munters]

If you push hard and the braking force is not enough you need more vacuum suport. Air in the system gives you the same force just later (the warmer the brakes the later the force because the air expannds). Bigger caliper piston=more force needed. Pumping brings pedal higher up but does not change the force.

[JLPurcell]

Still fighting this problem. I pulled the booster this evening and am sending it out to Brake Power Booster Exchange in OR. for check out and possible rebuild. I have also pulled the brake master cylinder and found that it is not the correct part. It is for the 70 - 71 models. I am looking for a NOS 1972 Master Cylinder if anyone has any leads let me know. I would like to have an original, if possible. I will be checking out the other components as I go through the complete system. 

 

[JLPurcell]

I pulled the Brake Booster and sent it out, it checked out OK. I put the stock front brake calipers and rotors on and bleed out the system and I still have a hard pedal and the front calipers are not releasing rear brakes are releasing. Experts out there? Master cylinder?