As the title says, the front disc brakes on my 92 G20 have recently started to lock up. It seems the pedal pressure is not releasing the front calipers completely. They're not completely "locked up" but they are tight enough to be overheating and causing issues after a few miles of city driving. I replaced the calipers and pads yesterday and all seemed good on the test drive. This morning on my way to work, about 15 miles, I could smell them overheating and the passenger side actually sounded metal to metal when stopping. After the van sat for a while and cooled off they seemed fine for a few miles but I barely made it back to my shop. My mechanic says it could be the vacuum booster or the proportioning valve not releasing the pedal pressure to the front brakes. This van also has an "isolation/dump" valve for the rear antilock system. What is the best way to isolate this issue? Any help is appreciated.

 

So only rear wheel anti-lock? The front brake lines avoid the ABS module? Is there a portioning / hold off valve plumbed in there? Rear drum or disc?

It is possible for brake lines to fail internally such that they act as a check valve. When you replaced the calipers did you also replace the rubber brake lines?

Have you replaced the booster or master cylinder? The plunger could be set too long causing the master cylinder to not retract all the way and not uncover the compensation ports. With the cover off do you see a bit of fluid movement when someone presses the brakes for the first less than 1/8" travel?

 

The booster and master cylinder were replaced about 3 years ago. There is a combination/proportioning valve. The front brakes are not controlled by the rear antilock system. This issue just started last week.

 

My guess is the rubber hoses. They swell internally and won't let the brake fluid return when you take you foot off the brake. Though since your last post was in June I'd say you probably figured out your problem by now.

 

Update: I disconnected the front brake line from the master cylinder and the calipers released. So, I replaced the master cylinder AND the front brake hoses for good measure even though they seemed to be fine. Definitely not 32 year old originals. Anyway, all is good now and I have about 200 miles on it since the repair. Brakes working great. I'm still amazed that the master cylinder locked up like that...new one on me. I would have never thought that could even happen.

 

Oh yeah, they can lock up like that for several reasons. Dry slide pins, clogged or pinched brake lines, corrosion or poor fitting slide tabs on the pads. There's tremendous force applied in squeezing them but not in letting them release. They're like a crocodiles mouth that can crush bone closing but a person can hold it closed with their hands. Anything that keeps them from operating smoothly will still allow them to compress but not release.

I had a neighbor, who moved here from the rust belt, bring me his mid 90s Silverado with a similar issue. Brake would lock up and slowly release over time. The rubber brake lines had a metal bracket crimped onto them to bolt to the body. The bracket rusted and swelled to the point it was pinching off the brake line. The braking system had plenty of force to push through the restriction but it would hold the pressure and slowly release it.