Hello to anyone reading this, I'm having some issues with the brakes locking up on my 1991 pickup (V6, 4x4, xtra cab) and was hoping for some advice.

I recently replaced the front rotors and pads, bled it, and it ran for a bit with no issues. Then the clutch / brake pedal bracket failed and it sat for a few weeks, in that time getting new rear shoes, cylinders (one of which was leaking pretty bad), and drums, all properly installed, adjusted, and spinning freely.

After getting it going again with a newly reinforced pedal bracket and well bled rear cylinders, I've taken it for a few drives and come to realize all four brakes are locking up with increasing intensity the further it heats up - this last time barely making it up the hill where I live and all four corners smoking after about 15 min of driving. It feels like the whole brake system builds pressure as it warms up and is unable to release it, as the brake pedal also gets pretty stiff, and everything loosens up after it cools off. I also noticed that pressing the brake pedal at idle drops rpms a bit, which didn't happen before.

Initial research tells me it could be bulging lines due to increased pressure (no more leaky rear cylinder?) not allowing anything to release, or an issue with the master cylinder / booster pushrod (which I'm a bit skeptical of because there was no issue there before). Either way I have no idea and haven't seen anything like this before, so any ideas here would be greatly appreciated.

Thanks in advance! (and apologies if this has been covered before, I didn't see anything)

 

Brake fluid doesn't expand with temperature, but water does. To make matters worse, brake fluid is hygroscopic, meaning it will absorb water from the air. (This is why you shouldn't use an old container of brake fluid.)

You can get a cheap "brake fluid tester." These supposedly guess at the water content based on the conductivity. Probably not very accurate, at least compared to a Brake Fluid Boiling Point tester.
But in your case, a complete replacement of the fluid might be the way to go. Remember that the bleeders are on the top of the brake cylinders, so just pushing new fluid from the master to the cylinders will leave old fluid in the "bottom" of the cylinders. You could do it in multiple steps, driving in between, to mix the new fluid with the old. Or remove the cylinders to turn them the right way. Or just do the best with what you're willing to do.

 

Yeah I see what you're saying, that would make sense. I'll try flushing everything and see what happens. Thanks

 

Ended up solving it - after reinforcing the pedal bracket the brake pedal was misadjusted, pressing the booster rod a quarter inch or so.