Brake Lock-Up

[Mark E.]

Re: Brake Lock-Up

The Delco 18M391429 remains available so I think I'll follow your lead and go that route.

$189 from Amazon Prime... in stock, free shipping, free returns to Whole Foods.

[Joe L.]

Re: Brake Lock-Up

Gentlemen: While I am not necessarily familiar with this or other later Corvette generations, I can relate a potential cause for this problem which I experienced with a foreign model car some years ago. I don't see that Joe mentions having removed or otherwise adjusted the MC prior to this problem occurring, but in my case I had either rebuilt or replaced the MC (don't recall which) and the very condition described started occurring almost immediately, (short drive, 5-8 miles and the brakes (front disc, rear drum) would lock up). Cool down for half an hour and all is fine until driven again. The problem? The MC push rod from pedal to piston was mis-adjusted too long semi-applying the brakes, and when they had dragged for a while, fluid was heated up and expanded and forced the brakes to apply. Readjust the rod to provide "lash" and all is well. As I said, it occurred when my MC was rebuilt or replaced but it would be worth checking to see if this one has sufficient free play in the push rod.

Rod------

Yes, I had not removed or adjusted anything prior to the problem. Also, in my application there is no separate or otherwise adjustable rod between the master cylinder and the power brake booster.

[Richard M.]

Re: Brake Lock-Up

I'm not familiar with the C4 Power Brake MC and Booster functionality, but is it possible the booster is related to the fault?

I ask because I had a identical "Brake Lock Up" issue on a Jaguar E-Type years ago. Was fine for a few miles, then brake drag. We'd have to stop for a cool down period, then continue. It's actually a very common fault with the Lockheed brake system of the era(1970's). Finally I was able to get back to the shop and didn't drive it until it was corrected.

As it turned out, the power brake booster internal vacuum port seal was leaking. This allowed the atmosphere side of the booster to activate, thus applying the brakes slightly. After a while when things got hot and expanded, the car couldn't move.

Another cause for a identical lock-up problem in the Lockheed design, was that the MC has a Air Reaction valve, activated by a small piston at the front of the MC. On pedal pressure, the piston moves out and activates the air valve for the booster atmosphere side. If that piston gets stuck when pedal is released, brakes get stuck. Also, if there's a hose leak between that MC air valve and booster, it could also lock the brakes. Crazy design. If something fails, brakes lock up. Could be dangerous under certain conditions.

Is the C4 MC and Booster a similar design?

To diagnose the booster, you might try disabling it and see if the brakes continue to lock on a test drive. You'd have to pull the vacuum hose and plug it. Of course it would be harder to apply the brakes with no boost, but it might help either confirm or eliminate the booster as the fault.

Rich

[Dave P.]

Re: Brake Lock-Up

When I was diagnosing the identical symptoms in my 1992 Y-car that Joe is experiencing in his 1992 Y-car, I thought of things like "hoses", " booster", "push-rod adjustment", even the ABS/TCS hardware as it is incorporated into the hydraulic circuits.

With the car in the air and the wheels removed, I found that I could pump-up the pedal with my foot, and the front rotors were impossible to turn by hand. I could apply the brakes, but they would not release. Having discovered I could duplicate the malfunction without driving it, it was now straight forward to diagnose. Lock them up, try something, and see if the rotors could be turned by hand. I pulled the master cyl back from the booster: No change, so not the booster or the push-rod adjustment.

Cracking either brake line at the master cyl released the front calipers. An imperceptible amount of fluid would come out of the nut, but the brakes would release. In my instance of this failure, it was the master cyl. Joe has the same car, same year, same failure scenario. A new master cyl fixed mine, it will very likely fix Joe's.

[Joe L.]

Re: Brake Lock-Up

I'm not familiar with the C4 Power Brake MC and Booster functionality, but is it possible the booster is related to the fault?

I ask because I had a identical "Brake Lock Up" issue on a Jaguar E-Type years ago. Was fine for a few miles, then brake drag. We'd have to stop for a cool down period, then continue. It's actually a very common fault with the Lockheed brake system of the era(1970's). Finally I was able to get back to the shop and didn't drive it until it was corrected.

As it turned out, the power brake booster internal vacuum port seal was leaking. This allowed the atmosphere side of the booster to activate, thus applying the brakes slightly. After a while when things got hot and expanded, the car couldn't move.

Another cause for a identical lock-up problem in the Lockheed design, was that the MC has a Air Reaction valve, activated by a small piston at the front of the MC. On pedal pressure, the piston moves out and activates the air valve for the booster atmosphere side. If that piston gets stuck when pedal is released, brakes get stuck. Also, if there's a hose leak between that MC air valve and booster, it could also lock the brakes. Crazy design. If something fails, brakes lock up. Could be dangerous under certain conditions.

Is the C4 MC and Booster a similar design?

To diagnose the booster, you might try disabling it and see if the brakes continue to lock on a test drive. You'd have to pull the vacuum hose and plug it. Of course it would be harder to apply the brakes with no boost, but it might help either confirm or eliminate the booster as the fault.

Rich

Rich------


Thanks very much for the input and your suggestion. I really do not know if the booster in my application is similar to the Lockheed design. However, my application is exactly the same as Dave's. After his detailed explanation of what he went through to diagnose his problem and correct it, I'm very confident that it will be the solution to mine, too. I have a new master cylinder on the way to me now.

One of the additional reasons I also suspect the master cylinder is something that I hate to admit but I will. Most of the brake fluid in my car is just what Bowling Green put in it 32-1/2 years ago. It's never been flushed and renewed. I have siphoned and replaced the fluid contained in the master cylinder once or twice but that's it. So, when I remove and "autopsy" this master cylinder, it's going to be interesting to see what the bore and pistons look like. It's also notable that the 2 times that this brake lock problem occurred, it was after the car had been undriven for months. That happenstance might have given bore/piston corrosion a chance to proceed to the point of causing the piston to "stick".

[Bill M.]

Re: Brake Lock-Up

Joe:

I had the same problem on my '65. Long story short: That tiny hole in the M/C was plugged with crud. That is a compensating port and allows warm fluid to expand back into the reservoir when your foot is off the brake pedal. If it is plugged, the brakes will lock up. At one point I had the L78 at full throttle in 1st gear and the car was grinding to a full stop! After it cooled down, the brakes released.


100_3457.jpg

edit: I forgot to mention that I cleared the crud by running a wire down the compensating port; problem solved, no more lock-up...