rhodus

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.

Triaged

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?

rhodus

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.