This happens when the brake booster push rod is adjusted out too far which does no allow the brake master to return fully holding the brakes on . To confirm the issue loosen the brake master mounting bolts 3 turns to see if the car starts to roll again, if so remove the master and adjust the booster push rod inward. Also a brake flex hose can come apart internally causing a short of check valve in the system but this is usually isolated to one brake either front or rear.
What kind of sound are you hearing?
Bleeding the system is not the answer. No air should have gotten into the hydraulic system if nothing related to it was opened or replaced. Also, even if there was air in the system, that can be compressed, and that prevents the brakes from applying properly and the brake pedal will feel low and mushy. That's the opposite of failing to release.
Given the age and mileage of the vehicle, a better suspect would be excessive debris built up inside the front calipers, behind the pistons. You have to push the pistons back in to their fully-retracted positions to make room for the new, thicker brake pads. It should be easy to pry them back in with a screwdriver before the calipers are removed. If you have to resort to using a c-clamp to reset the pistons, which should never be necessary, it's because of that debris. Even if you can get the new pads installed, you have to stroke the brake pedal multiple times, (never ever more than half-way to the floor), to run the pistons back out to adjust them. They will do that, but they can still be hung up in that debris.
There's a "square-cut" seal around the piston in each caliper. When you press the brake pedal, the piston gets pushed out a little, and that square-cut seal wants to stick to it. The seal bends a little, then, if necessary for the continual self-adjustment, the pistons will slide through the seal just a little. The important part is when you release the brake pedal, that seal wants to straighten out. In so doing, it pulls the piston back a few thousandths of an inch. That is all that pulls the brake pads away from the rotor to prevent them from dragging. The rust, scale, and rubber chips that form that debris that builds up over time reduces the seal's ability to pull the piston back. When that gets bad enough, the pads can get hot enough to start smoking after a short drive. The additional clue is if you stop on a slight incline, shift to "neutral", and release the brakes, the car will not creep downhill on its own.
There can also be a ring of rust built up around the piston. That always occurs outside of the square-cut seal, opposite of the side where the brake fluid is held in. That rust doesn't cause a problem until you reset the piston for the new pads. Pushing the piston back in runs that ring of rust under the seal. Most of the time that will not cause brake fluid to leak out, but it will make it impossible for the seal to retract the piston when you release the brake pedal. This is another case where you may not be able to pry the piston back in easily. If you can only get it to retract with a c-clamp, both calipers must be replaced.
Back in the '70s and '80s it was common to rebuild calipers and wheel cylinders as part of a professional brake job. The kits cost less than ten dollars, and if a replacement piston was needed, those cost around $20.00. Professionally-rebuilt calipers with a warranty usually cost up to $90.00. That's why rebuilding them in the shop was the better value for the customer. Today you can find rebuilt calipers from the auto parts stores often for as little as $20.00, with a warranty. We can't rebuild them ourselves that inexpensively, so now this is the better value. Always replace calipers in matched pairs to maintain balanced braking.
Here's a link to a related article:
https://www.2carpros.com/articles/brake-caliper-replacement
Also check out this video to see if you overlooked anything:
https://youtu.be/mOyHFT0hoKc
They're using the pry bar as I described to retract the pistons.
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Monday, May 18th, 2020 AT 3:46 PM
