Front Brakes Locked - Seeking Advice

[ScottZ95ZR1]

Both front brakes locked up on my '92 and upon opening the right front caliper bleeder, both wheels freed up..... so I think this indicates I have a hydraulic issue verses mechanical problem within the brake calipers.

Brake hoses were upgraded to steel-braided lines back in '95 so these are not the old rubber hoses. And it seems very unlikely that both sides would fail at the same time. I think failed hoses are not my issue but I have ordered new stainless hoses from DRM.

I flushed brake fluid by pulling 2 quarts of fresh DOT-4 through system using a compressed air vacuum tool. After bleeding the calipers (RR, LR, RF, then LF) using same tool, I had good firm brake pedal that would hold for a couple minutes without softening. I put wheels back on car for short, neighborhood test drive.

Brake pedal remained firm (felt normal - GOOD!) and I felt ABS do self-test as car accelerated. Brakes felt and seemed to function perfectly at first.... but after applying brakes several times within short distance of about 100 yards, I felt brakes start dragging again so parked her. Both sides drag then lock up equally; the car never pulled to either side and brake light on DIC is not illuminated. At this point the pedal is hard (no longer just firm).

With key off I have hard pedal and when engine is started, I feel brake pedal soften a bit as expected. I think this indicates my Brake Booster is still good, so I next moved to the Brake Booster Check Valve. Unfortunately, the check valve appears to be good; I am only able to blow through intake end with no flow from booster side. I never thought I would be so bummed out by a suspected part testing good. 😊

I have sourced a new Master Cylinder but think I'd experience soft/spongy pedal if that was my problem.

Not sure where I focus next and welcome feedback/advice from the many here smarter than me......most everyone here qualifies!

Thanks in advance for your help.

[Mikey]

Maybe an issue with the ABS controller?

[ScottZ95ZR1]

OMG.

After chasing this for a month I think my floor mat was interfering with brake pedal travel. I hope to confirm (admit) this was my issue in the morning.

I should pay closer attention to FSM details. When I recover from the "why did I not find this sooner" humility I am feeling tonight I will at least have fresh brake fluid.....

[ScottZ95ZR1]

Originally Posted by Mikey

Maybe an issue with the ABS controller?

Thanks Mikey. I don't have a clue but may be time for me to learn about ABS.

My plan is to replace Master Cylinder next. I do not have Tech 1 and think that will be needed if issue is with ABS

[-=Jeff=-]

It sure I can offer anything yet, but wanted to say HI

[ScottZ95ZR1]

Originally Posted by -=Jeff=-

It sure I can offer anything yet, but wanted to say HI

Howdy Jeff - hope all is well for you and your family. I hope to visit Chicago in the not too distant future and will reach out when that happens.

I retired two months ago so time to have fun!

[Thomas ZR-1]

DOT3 is the correct fluid...not DOT4. DOT4 is chemical much more aggresive so the rubber seals in the master cylinder are swelling and so: system blocked. Release the 2 nuts that are connecting the master cylinder to the brake booster and you will see: the systems is unlocked again...but don`t try a test drive with an unfixed master cylinder.

[ScottZ95ZR1]

[/QUOTE]=Thomas ZR-1;345680]DOT3 is the correct fluid...not DOT4. /QUOTE]

I understand DOT3 and DOT4 fluids to be glycol-based and compatible, with 4 having a bit higher boiling point. I think it is DOT5 that should not be used in our cars.

But I do understand you are correct that DOT3 is what's 'called for', and it is more than sufficient for cars not seeing track duty, which mine does not.

I also think you are correct that my master cylinder is the culprit and will replace it next.

Thanks very much for your feedback Thomas.

[ScottZ95ZR1]

Bummer. Brakes locked up on second test-drive attempt.

First short trip everything seemed perfect. Two hours later I could not get out of driveway. I am perplexed.

[hotroddaddyo]

Any suspicion of the flexible brake lines at the master cylinder?