help with brake problem

[John C.]

Anybody out there good with brakes?? I just aquired a '65 convertible with power disc brakes. The previous owner had replaced everything in the braking system except the master cylinder.
The problem: brakes work perfectly when initially taking the car out onto the road. No pulling to either side, smooth, normal pedal depression pressure, etc. After about 15 minutes of driving, the amount of pedal pressure required to apply braking diminishes, in other words, instead of the normal 1-2" of pedal depression required to activate braking, now that has changed and it only needs barely touching the pedal to brake the car. Eventually, the brakes are "applied" with no pedal pressure at all. As if you are driving with the pedal pressed while driving. I turned around and took the car home ASAP, pulled into the garage with the right front smoking lightly. I jacked the car up and checked each wheel to see which ones were being affected. As it turns out, all 4 wheels were under brake pressure.
I waited about 30 minutes and checked again, all 4 wheels were turing freely. Wierd.
Common sense says either replace or rebuild the master cylinder since that is the only part of the system that hasn't been rebuilt. Never-the -less, I would still like to hear someone elses opinion on this.
My question is why does the car function properly when cold and get worse after driving for 15-20 min, then free itself up after sitting for a while?
The master cylinder is a dual chamber model. Is this model supposed to have a preportioning valve. The front bowl feeds the front and the rear bowl supplies the rear.
Help!!

[Richard M.]

Re: help with brake problem

If the rubber fluid lines are new, then I would suspect the booster, or some relationship to it. When you pull the check valve off of the booster what happens? Check it when the brakes lock up too.

Also, there is a clevis arm on the booster rod which connects to the brake pedal. Check to make sure that is not threaded out too far, so much that it will put tension on the booster to apply the brakes. I doubt it's that but worth a check.

I don't know if it's possible with the GM booster, but I've seen PB systems go into "self-application" mode if their is a internal leak in the booster between the vacuum side and the atmosphere side.

It just seems like the brakes are simply being turned on via the booster. You could remove the vacuum source at the booster and take it out for a 15 minute ride and see what happens.......away from traffic on easy flat roads for safety. You'll need extra brake pressure without assist so be cautious. If it still locks up the problem is likely hydraulic. If not, I'd say booster.

Rich

edit: I'm thinking of 67 brakes as I just did a PB conversion on one. I'm not sure if the 65 system has major differences, but I think the basics are the same.

[Timothy B.]

Re: help with brake problem

John,

Do some reading in the archives on this problem as I remember it being discussed in the past.

If this master cylinder has been overhauled it may be possible a check valve was installed. The drum brake cars had this valve but the disc cars do not. The valve may be part of the overhaul kit and could have been installed by mistake.

On drum brake cars, the springs pull the shoes back from the drum. On disc cars there are no springs to pull the disc pads so fluid is allowed to pass back to the M/C.

[Bill M.]

Re: help with brake problem

John,

I had exactly the same problem on my PB '65. In my case, the rear compensating port in the master cylinder was plugged.

If you look into the fluid reservoir, the compensating port is the small hole to the front in each reservoir. If all 4 are locking up, the compensating port in your rear reservoir may be plugged.

The compensating port allows expanding (due to heat) brake fluid to flow back into the reservoir. If it can't expand back into the reservoir, it applies the brakes, which generates more heat, which expands the fluid more, which applies the brakes more.

I wound up at full throttle in 1st gear and the car just ground to a halt.

I would get it to lock up again and pull the master cylinder loose from the booster to see if that fixes it. It is possible that your master is not fully retracting and the piston seal is covering the comp. port. If that doesn't fix it, try running a paper clip into the port to clear a clog. That fixed it for me.

Bill

[Claus S.]

Re: help with brake problem

If you have you PB the hose from the booster may have come loose?
I suppose that you have checked that?

[Don H.]

Re: help with brake problem

John-
there is always something with these cars, but don't you just love this tech board? There is nothing that can happen with your car that someone else has not already dealt with. If you can only have one car in your stable, a 65 convertible is a good one to have...
Welcome aboard !

[John C.]

Re: help with brake problem

If you have you PB the hose from the booster may have come loose?
I suppose that you have checked that?

Yes, not only is it connected, the hose is new. What really gets me about this whole thing is the fact that the brakes work fine at first, then begin to act up after driving for a short while. Then the problem clears itself up after leting the car sit for 30 minutes. Makes you think the problem is somehow related to a heat problem. Something changes after things heat up a bit.

[Michael W.]

Re: help with brake problem

Dupe?

https://www.forums.ncrs.org/showthre...83343&uid=9806

Next time it happens, loosen off the two nuts holding the M/C to the booster. if the brakes release, it's the M/C at fault.

[John C.]

Re: help with brake problem

Dupe?

https://www.forums.ncrs.org/showthre...83343&uid=9806

Next time it happens, loosen off the two nuts holding the M/C to the booster. if the brakes release, it's the M/C at fault.

Well, in response to that, let me say this. As the problem worsens while driving, the amount of pedal depression distance/ the typical 1-2" usually needed to apply braking, changes to the point where you can barely touch the pedal and braking occurs. I think this verifies your thoughts, that the piston shaft in the master cylinder isn't releasing and by loosening the M/C, you are doing what the M/C should be doing by itself. The same thing would happen if the pedal was disconnected.
Thanks buddy!!
One more question; would you replace the entire unit or rebuild it? I realize some things on the car have date codes and some don't. My car is stock with the exception of the transmission. It has the correct engine, but not the correct numbers matching tranny.

[Don H.]

Re: help with brake problem

John, drop down a few threads in this forum and see answers posted the first time you ran this question. You might find the right answer there....

[Michael W.]

Re: help with brake problem

I'd rebuild it after finding the root cause. M/Cs are not complicated.

[John L.]

Re: help with brake problem

John:
Obviously the MC is not fully releasing (to uncover one of the orifices to the reservoir) once the vehicle/system is at operating temp. The items to check are, 1 the interface between the MC and Power brake booster for a mismatch of parts; or 2 the most likely cause, an incorrectly adjusted pushrod or incorrect part. page 5-11 in your service manual, or 3 missing springs or stuck pistons in the MC itself.

[John C.]

Re: help with brake problem

John-
there is always something with these cars, but don't you just love this tech board? There is nothing that can happen with your car that someone else has not already dealt with. If you can only have one car in your stable, a 65 convertible is a good one to have...
Welcome aboard !

Thanks Don, yes, the 100's of years of knowledge is great. Can someone out there clarify something for me. In an attempt to buy either a rebuild kit or a completely new master cylinder, I discovered the 65 wasn't available with Vacuum assist brakes, only hydro assist. Is this correct??
My car has vacuum assist 4 wheel disc brakes. I was told that this wasn't available until 67. There is no preportioning valve, and as I understand it, a car with a dual chamber master cylinder has to have a preportioning valve to function properly ?????.
Now I'm stuck trying to repair a problem with braking components that didn't even come on my car, a car that was supposed to be bone stock. Needless to say, I'm not happy.
I think you guys are probably right in that the master cylinder is bad. I guess the thing to do at this point is replace or rebuild it and install a preportioning valve. Would you guys agree???
JC

[Michael W.]

Re: help with brake problem

I think you guys are probably right in that the master cylinder is bad. I guess the thing to do at this point is replace or rebuild it and install a preportioning valve. Would you guys agree???
JC

Couldn't disagree more- you've been given a truck load of incorrect information on what was available in '65. Sounds like some of the info that's given out on 'that other' website. Frightening.

Vacuum assisted brakes (option J50) have been available on Corvettes since 1963 and haven't changed much since then. No Corvette before 1978 had a proportioning valve aside from a very few J56 equipped cars and it is of no benefit on non-J56 cars. No Corvette came with hydro assist brakes. Corvettes in later years had a dual circuit master and brake line, but that's irrelevant also.

I'd also suggest that you choose one of your two threads on this subject

https://www.forums.ncrs.org/showthre...83342&uid=6540

and have the moderators close or merge the other.

[John H.]

Re: help with brake problem

Thanks Don, yes, the 100's of years of knowledge is great. Can someone out there clarify something for me. In an attempt to buy either a rebuild kit or a completely new master cylinder, I discovered the 65 wasn't available with Vacuum assist brakes, only hydro assist. Is this correct??
My car has vacuum assist 4 wheel disc brakes. I was told that this wasn't available until 67. There is no preportioning valve, and as I understand it, a car with a dual chamber master cylinder has to have a preportioning valve to function properly ?????.
Now I'm stuck trying to repair a problem with braking components that didn't even come on my car, a car that was supposed to be bone stock. Needless to say, I'm not happy.
I think you guys are probably right in that the master cylinder is bad. I guess the thing to do at this point is replace or rebuild it and install a preportioning valve. Would you guys agree???
JC

John -

Can you post a photo of the master cylinder and booster so we know what you're dealing with? All Corvettes with power brakes use a vacuum booster; none ever used a hydraulic booster, although some have been converted to an aftermarket "Hydroboost" system that gets its power from the power steering pump.

4-wheel disc brake Corvettes prior to 1977 or so don't use a proportioning valve - proportioning is designed into the system via different bore diameters for the front and rear caliper pistons.

If your master cylinder and booster are factory, they'll look like the photo below.