Stock brakes on an '86 Iroc Z. Master cylinder looks like it may have been replaced once but can't tell for sure. Not sure what the factory one looked like, but this one looks like it's got a newer fluid reservoir and an aftermarket cap on it.

Anyhow, pads, rotors are all in decent shape. Fluid looks good- both level and clarity. The brakes don't feel bad.

Problem is, the front calipers randomly stick. I can tell because when going down the freeway, it starts to feel like I'm towing a small boat, and the front end starts to shimmy as the calipers heat up. Sometimes the brakes smell, sometimes not.

The rubber hoses appear to be original; so I replaced those, flushed a bunch of fluid through while bleeding the brakes. Kept the reservoir level up the whole time.

So I went to do my victory lap, and....no change. Damn things are still sticking.

I'd like to check/diagnose stuff versus just pulling and replacing parts. What should I look at next?

 

is it both calipers sticking? usually it's 1 sticking, the car may pull that way. when you get out feel the rim. it may be much hotter than the other side and smell of burnt brake pad material or even burnt wheel bearing grease.

 

It's both. Before and after replacing the brake hoses. I'd check with the back of my hand and both front wheels were scorching hot. The car doesn't pull it just gets really heavy when it happens. Rear wheels are cool/ normal road temperature.

 

hmmm, IDK if the master cylinder could stick? i'd bet its your calipers. did the car sit for a period of time?
you could try pulling the calipers off and working the pistons back and forth, (use old pads, then c-clamp). i dont think calipers are that expensive but i understand not wanting to waste money and find the exact problem.

 

Stock calipers are dirt cheap. $20 each at my local Autozone. I know they generally don't go bad, but since you said it's both fronts and the rears are cool, I'd look at the proportioning valve. It may not be providing enough pressure to the rears, forcing the fronts to do all of the braking.

 

Replaced both front calipers today, brand new pads. Bled the system again.

Got everything bled, started up the car to make sure the brakes felt OK with the booster running.

Front wheels are locked up tight again after pumping the brakes a few times, seems they still won't release fully. Can't turn the rotors by hand.

I did notice while bleeding (the brakes, not me) that the rears would barely eek out any fluid while bleeding. Not like the fronts. The fronts would gush. The rears, maybe a teaspoon. I don't know if that's normal or not?

SO.... what do you all think is next? I've heard about this proportioning valve...where is it? can it be replaced?

Or should I just go for the master cylinder? I'm wondering if the one that's on there wasn't bench bled correctly or something. I have no idea if that would cause the issue I'm having but at this point I just want the damn car to stop when it should (and not when it shouldn't) at this point!!

 

Both front and rear calipers should expel the same amount of fluid when bleeding. The proportioning valve is the aluminum block right under the master cylinder. I don't think an incorrectly bled master would cause your problem. I've had that happen to me in the past, and it felt just like when a caliper is not bled properly, i.e. spongy, low pedal.