1968 Master Cylinder

[Kurt B.]

Re: 1968 Master Cylinder

Don,
The rotors were replaced years ago, and no problem. Then the car was not driven for many years. All was well until AFTER I had the master cylinder reslevved and rebuilt. I do get a very firm pedal, a perfect pedal, after I have driven the car for 15-20 minutes and it stays that way until I park the car for a few days. Then the problem manifests itself again. This is why I don't believe it has anything to do with rotor runout or my calipers, since once the car is driven for a while, the brake system is perfect and will stop on a dime like a new car.
It is almost as though the master cylinder or something inside it, must warm up before functioning properly. It's mind boggling.
Kurt

[Michael M.]

Re: 1968 Master Cylinder

Kurt, I have read your thread all the way through as well as your previous threads. Instead of going out and purchasing a correct master cylinder to replace your current one, you may want to simply purchase a replacement from a local auto parts store and install it on on your car. If it solves the problem, you can have your current and correct master cylinder rebuilt again (by a different vendor I presume). Although parts changing isn't always the recommended method, doing it with less expensive parts to eliminate/confirm the issue may be in order here. It seems you are not 100% sure what the problem is (and neither am I frankly) and so you may be able to save yourself some time and effort by using a less expensive (and readily available) master cylinder as a temporary replacement part. I would hate to think that you tracked down and paid top dollar for a factory correct master cylinder, paid for a rebuild, and then only to have the same symptoms. Also, if you suspect that the master heats up and swells seals, etc. thus providing a good pedal, is it possible that same logic could be applied to the booster? Just a thought.

[Kurt B.]

Re: 1968 Master Cylinder

Michael,
Good suggestion.
I sure hope it has nothing to dowith the booster as that is a royal pain in the neck to remove and replace.
I thought I read somewhere that to test the booster, it should be a very frim pedal with the motor off and therefore no vacuum assist. My booster is rock hard with the motor off so I am assuming this confirms that the booster, which was rebuilt by Booster Dewey (from Hemmings Motor News Magazine) is OK.
I will do a swap over the winter with a regular auto parts master cylinder as a test. I still have a few good driving days in the fall foliage of New England and since this is the first year I can drive this car in 10 years, I want all the driving time I can get before winter storage.
Kurt

[Michael M.]

Re: 1968 Master Cylinder

Kurt, enjoy the car but be careful. If you know you have a brake problem, I would be wary of any spirited driving and you never know when an emergency stop may be necessary. If there is indeed a seal problem in the master, it could let go completely without warning. It shouldn't take more than a couple of hours to remove and replace the master and blled the brakes. If that fixes the problem, you will have trouble free driving for as long as you want until you get around to getting the original repaired. No one will be judging your car when your flying down the road. You just need good brakes?

[Kurt B.]

Re: 1968 Master Cylinder Update

I just spoke with the vendor. They had tried to reply to my e-mail but are having problems with Verizon e-mail addresses (which mine is). They had several last week. I'm glad I called and spoke to a human. (Probably should have done that to begin with)
I was told to remove the brake lines at the master cylinder and plug the outlets on the m/c and then I should have a rock solid pedal. If I do not, then the M/C is the culprit.
They seem to feel that the problem is at the wheel calipers but I am not convinced of that.
They also looked up my invoice and said that they only resleeved it, and did not rebuild it. SURPRISE (My error, I thought they had rebuilt it as well) So perhaps I need a rebuild. I was told that the bore is standard size and that a standard rebuild kit can be used. So now I have some things to test and perhaps the vendor did nothing wrong. It may be as simple as that I need to rebuild the master cylinder since I thought they did it and according to them today, they did not.
The saga continues.
Thanks for all the input and suggestions
Kurt

[Norm C.]

One More Possibility

Kurt....

I did a complete brake job earlier this summer on my C3. I kept all my own calipers and MC. White Post did the restore on the MC,,,,but I'm almost sure they used the bronze sleeve for the same reasons listed above regarding the trade offs with stainless.

It was suggested by two very knowledgeable Corvette mech's to also do the following two tips. 1. Jack the rear corners in the routine order (RR,LR,RF & LF) when bleeding to get trapped air out 2. Disconnect the lines on the "out" side of the porportioning valve and open all the bleeders at each caliper and blow out all the old fluid. (needs to be done with the lines AFTER the valve so as not damage or alter the valve settings) I did this and the first time I had factory level firmness.... something that had been missing for along time. Rotors appeared to be in excellent shape so didn't deal with those or "runout"

I too echo the need for a "complete" rebuild on the MC. Seals and plunger have as much to do with performance as the sleeve. White Post will do either an "original factory finish" or a "driver" finish,,,, which is what I did to retain the original paint on the MC. I used White Post because of the many strong recommendations I received from here. Good Luck,,, be sure to finish the story when you get it resolved!!!!

[Chuck S.]

Re: 1968 Master Cylinder

Uh...Unless it says that in the service manual, I would intuitively disagree with your conclusion. Removing the vacuum feed from the booster only serves to remove the booster function from the equation, not that it's working correctly. A hard firm pedal without vacuum to the booseter should indicate that the hydraulic system, i.e. the MC, is working as expected.

[Geoff C.]

Re: 1968 Master Cylinder

Kurt,

One other thing to check is to ensure that the PB booster front rubber seal is not missing. This inch and a half or so OD thick looking rubber washer is in the depression in the front of the PB booster. It's about 1/8 inch thick and seals the PB to the M/C. When missing, it allows the constant vacuum of the PB to suck brake fluid past the rear seals in the M/C. From thence it attacks the PB booster internals as well as being sucked into the intake.

Over 2 decades ago when I performed rebuilds on PB Boosters, I often found a pool of half a cup of brake fluid inside the booster itself. Then Nationwide decided that they did not want economically underwrite liability for that service and thus we discontinued that Rebuild service, even though we had all the Kent Moore service tools.

Geoffrey