I have a 74 BB convertible that I'm having aggravating issues with. Front caliper pistons will not stay out against pads and withdraw back into cylinders. Second new Master cylinder (non-power car and bench bled), new pistons and o-rings, new pads, new front rubber hoses, Dot 3 fluid. Calipers and all lines appear new as well as rotors. Have had car about 2 years and have vacuum bled, pressure bled, old style pump and hold bled. Pistons move back and then brake pedal has to travel 2/3 before pads squeeze the rotors. Have another 74, a 70 and a 66 Vette with o-ring calipers and have never encountered this before. I even pulled the piston springs to make them longer in order to keep pistons in contact with pads. Any advice would be greatly appreciated.

 

The only thing that comes to mind is the imperative gap between the booster rod and the piston in the MC. I suspect the gap is too large.
On the opposite side of this gap specifications story; if the gap is too tight the pads will not release or in laymans all four brakes drag going down the road.
But in your case I would suspect the gap is allowing the MC piston to move farther rearward than normal.

An earlier post on this gap specs last summer I believe was 0.060 gap. There was even a special tool that measured that distance for proper set-up.
So the question becomes:
is the MC a Delco or aftermarket?
is the booster rod adjustable?
How do you check the booster rod gap without buying the special tool? Silly Putty. By removing the two MC mounting nuts and carefully sliding the MC forward, (you should be able to do this w/o removing the lines) place a dab of putty at the end of the MC piston. Reinstall MC, tighten nuts, remove MC again, carefully remove putty, check the thickness. That is your gap.

 

The MC is a retro with numbers matching from Zip (5455509). Yes, the rod is adjustable. It was way to tight when I first bought the car and the MC was for a P/B set-up. I knew the gap was small, backed it off about one turn, but wasn't aware that the gap could cause this problem if it was slightly too wide. I have checked many exhaust/intake valves with putty so I will run a check on this gap the same way. Thank you so much. Greatly appreciated! I haven't driven the car very much as it never felt very safe, The brakes could never be locked up no matter how much pressure was applied.

 

The rod backs away from the master cylinder piston. Should have no effect. The spring under the master cyl piston must be too stiff .Pushing master piston away and sucking fluid back pulling wheel cyl pistons in. You would think master would pump up. I think master is the bad guy here.

 

There was a poster on here last year with a similar problem. He had bought a non-PB master for a PB car. (or vise versa). Anyway, he posted a picture of the two masters side-by-side showing the mount flange & piston. What a HUGE difference where the piston was in relation to where the booster rod would contact. It was obvious one of the masters was not going to work.

 

If the rod doesn't back away from the master cylinder piston (i.e. stuck somehow) then the fluid will do whatever the brake pedal does. I don't remember but would have to guess that there's a spring that pulls the pedal back and therefore pulls the pistons back.

 

Sorry, just re-read your post and answered my own question.

 

This occurs in the shop where you can see it happening or it happens when driving?

You did check and minimize the rotor runout?

 

This is a great question, I bought a motorcycle that had a slightly bent front rotor. The guy said the brakes needed to be bled, I did that and it didn't help at all. It was quite dangerous, took two full squeezes of front brake lever to get to catch. After some time I realized the rotor was slightly bent, I straightened it and the brakes worked fine after that. The brake pistons were being pushed too far back from the bent rotor.

 

First off I thought your car was NON-power brake car but comments are being made dealing with the booster???

Not that it matters... but a bit confusing.

I would put a brake system pressure gauge on the steel line that your flexible hose attaches to that is going to the caliper and see what your brake fluid pressure is.

When you worked on the piston/seals of the caliper ...were there any spring behind the pistons????

NOT saying that you did it incorrectly or not but when bench bleeding master cylinder it is WISE to NOT go more than 1 inch of piston travel when bench bleeding it. Some master cylinder have instruction that they may only want you to go 3/4" .

I also feel that the master cylinder may be the culprit due to if you have bled the system as how you stated you have then there should be no air in the system...even if you bleed to again. which is why I mentioned doing a pressure test on how much pressure the master cylinder is actually making.

Knowing that the front brake apply first when you ropes the pedal. Have someone there turning the rotor while someone slowly presses on the brake pedal and when you begin to feel a drag. Have that person tell you how far they went with the pedal ...IF you are trying to get the pedal to begin to make the caliper engage sooner.

IF so...even though it is not normal and may be just for testing purposes...extend out the adjustable rod and have it slowly begin to press in on the piston of the master cylinder and test to to see that IF you extend the rod out,...the front calipers and pads are NOT beginning to press again the rotor. THEN see how that does in regards to the the amount of pedal travel. You might be surprised on what you find occurs. Even if you just adjust the rod so it has almost NO end play and just touches the back of the master cylinder.

And just saying that hopefully all of the brake bleeding was done when the master cylinder was LEVEL and not at an angle.

YES...rotor run-out is important....but that is not going to effect the amount of brake pedal travel when sitting still testing it. The rotor run-out can effect the system after it has been used and the rotors have turned countless times.

DUB

 

It is a non-power car and this is the second new MC I have tried. Both were bench bled on the level.

There were springs and I tried bleeding with springs out (as some forums have suggested) and swings in. No difference.

No instructions with either MC advising how far to push in rod so pushed them in all the way.

I am at the wheel while another person pumps the pedal. The rotor is not moved at all when this occurs and they are new rotors.

I took a thin knife and held the piston out when the brake was pushed and it stayed until the pedal was pushed again.

The pedal has to be pressed at least halfway before pistons contact the pads. The a little further until pads squeeze rotors.

I will experiment with the adjustment on the rod and see what happens. Thank you for your input. Much obliged.

 

Occurs in the shop while I'm looking at it without spinning the rotors. Rotors are new.

 

That makes sense, but these rotors are new. Pistons move back in without rotation of the rotor.

 

I assume the master cylinder is for a NON-power brake car .

I can tell you that I can see the pistons relax once the pedal is released as it should. So it is hard to say without being able to see how much yours are pulling back.

This is why I am not a fan of the 'O' ring seals and much rather have the lipped seals due to it can be possible that the piston is rocking on the seal and the seal not moving outwards any more due to brake fluid pressure...which does not occur on a lipped seal.

Not wanting to get into it the debate/discussion why your other cars are all okay if they all have 'O' rings are not doing it.

Because a hydraulic system is what it is and if the pressure is what it is supposed to be and all of the considerations are correct...you should not be having this issue...and IF the issue is in the excessive piston movement....what are the two factors that make the piston move. Brake fluid pressure and the ability or inability of the seal that keep it under pressure and allow proper movement of the piston.

The only reason I am saying this is that I have dealt with so many brake issues over they years on these cars that sometimes what may be fine on one will not work on another. I have lost count on swapping parts from one car to another to TEST and VERIFY that what I feel is bad is actually bad even though it makes no sense.

For what it is worth...a real world scenario.

I was putting on a power brake booster/master cylinder on a 1972 with manual brakes and I could NOT get it to have a good peal and they were all new parts and I pressure bled it with about 2 gallons of fluid. And I even tapped on the frame when bleeding it to shock it so any air bubbles stuck to the sides of the brake lines released and would flow out when I bled it. So....there was no air in the system. I took out my GOOD power brake booster and master cylinder combo out of my 1977 and installed it in that 1972 and I had NO BRAKES. The brake pedal went to the floor. When I put the new assembly in my car that was just on the 1972...I had an awesome hard brake pedal. It took me a while to figure out what was wrong and I had to modify something to get the brakes to work like they should on the 1972. Most people would feel that just does not make sense but my modification WORKED and the owner never had any brake issues..

DUB

 

I give up DUB, what was the mod?

GUSTO

 

Rustavo,
You say you had no instructions with your MC on bench bleeding procedures. My new master came in a box which was tossed to the side. I bench bled the MC. A day later I noticed on the box in somewhat huge red letters. WARNING: DO NOT PUSH PISTON IN MORE THAN 1 + 3/8" or you will void the warranty and a return of the MC will not be accepted.
I ruined the piston seals. If you shove your screwdriver in the MC too far, the lip of the seal will roll over, damaging it and voiding the warranty. And the pedal will go to the floor later on. Lesson learned. Expensive lesson learned. Stupid lesson learned. Papa always said; Read The Instructions.
( I have some pictures of the proper procedure in my profile album)

 

So if I did damage the seal while bench bleeding, would the system hold pressure for months without ever going to the floor? Pedal might go to the floor, but I would have to shove it down. I have driven the car with this brake issue, but not much. Pedal feel never changes during the weeks/months. Maybe I will buy an inexpensive MC locally and try the 1" piston push. Thank you for your input. I have a couple of things to try from folks helping out on this forum. Really appreciated.

 

Actually according to the MC Vette vendor its 1 + 3/8" stroke, with a phillips screwdriver. NO MORE. No less.
I was just giving you ideas. I have never taken apart a MC to verify the piston seal rolled over. However, I can see this happening on either the front reservoir only or the rear reservoir only, depending on the design of the piston in the bore. If only one of the two pistons seals rolled over, then you would still have good brakes on half of the car anyway. Take a look at my profile, brake bleeding album.

More food for thought. The Proportioning Valve splits the brake circuits from front to back and on the front it splits left & right. I wonder if that valve is not allowing the proper fluid- pressure to the front?

 

OK, no knock-back when you are driving.

I'd guess it's one of two issues.

#1 - The master cylinder has lip seals inside. When the pedal is released the lips are supposed to fold in enough to let fluid go past them so they don't pull a vacuum on the lines which then pulls the caliper pistons back. Maybe your master has a lip seal that are too heavy or a spring and washer that is against the outer lip enough that it can pull a vacuum.

#2 - Something is different with your o-ring piston setups compared to most, and the o-rings are rolling into a loaded position so that they work like a spring and push the pistons back as they unroll once the fluid pressure is released. You saying you can hold the pistons when the master is released and they stay out tends to point to this not being the issue.

There must be clearance between the pushrod and the master piston. It shouldn't be too hard to determine how much you have by carefully moving the pedal and feeling when contact is made.

 

Ok. I'm gonna take the MC apart and check the seals. Then reassemble and go with the steps in your album.