All brakes gradually lock up as you drive?

DONALD OGLESBY

MEMBER
1970 OLDSMOBILE CUTLASS
5.7L
V8
2WD
AUTOMATIC
50 MILES

I have the car listed above that has front disc brakes and rear drum brake. I replaced the rear drum with disc brakes along with a new 1 1/8” bore master cylinder and a new Disc/Disc proportional valve. The brakes seemed to work fine but I noticed that it was taking more RPMs to drive. After about 15 minutes all four brakes lock up. I am going to replace the proportional valve but that does not work what do you recommend. It drives fine when it is cold or just started up but by the time it warms up the brakes gradually lock up. It is not air in the system. I don’t know. I need help.

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
48,323 POSTS

This happens when the brake booster is not returning fully, or the brake booster push rod is adjusted out too far. To confirm the problem when the brakes are locked up loosen the brake master cylinder three turns to see if the brake release and if so, replace the booster or adjust the booster push rod inward. The proportion valve will not cause this problem.

This guide can help as well:

https://www.2carpros.com/articles/a-comprehensive-guide-to-fixing-automotive-brakes-locking-up

Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.

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Sunday, April 30th, 2023 AT 11:46 AM

DONALD OGLESBY

MEMBER
2 POSTS

Are you speaking of the Rod that attaches the brake pedal to the booster? The only adjustment is under the dash where it attaches to the brake pedal. It has a nut on it. Very little adjustment at the pedal. No adjustment at the firewall in the engine compartment.

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Tuesday, May 2nd, 2023 AT 9:34 AM

KEN L

MASTER CERTIFIED MECHANIC
48,323 POSTS

Thanks for the images. The rod I am talking about is between the booster and the master cylinder. Did you try to loosen the master mounting bolts to see if the system depressurizes? If not, the master cylinder is bad, and you need another one.

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Tuesday, May 2nd, 2023 AT 1:03 PM