Brake drag/lock
 

Hi all-

I can use some help and advice! I have an 87s4, and I've been having some very frustrating problems with my rear brakes. It started by rear brakes locking up to the point where the car would not even move! At this point I have changed out the rear brake lines, bias valve, master cylinder, and brake booster! I took the car out today and in 5 minutes both rear brakes locked up again! I am completely baffled by this, I have bleed the brakes multiple times. I could use some sound advice at this point. Thanks.


Dan Gates

Dan,

What brake fluid did you use? Are the pistons moving freely in & out? Brake pads moving freely?

Aryan- I used blue ate racing. As far as the pads and pistons, I'm assuming they are?! Thanks,

Dan

you need to check the release point of the pushrod, chances are that the push rod doesnt have enough free play or any for that matter.
The MC will then not be able to release the fluid and thus with partial load on the pads they will overheat and boil the fluid and lock the brakes

In the 5 mins you drove the car are you using the brakes much?

Hows the free play on the E brake?

sounds as if you may need to rebuild the rear calipers and ensure the pads are moving freely.

How do I check the clearance on the push rod? I will try that first then consider the calipers. Thanks

Dan

if you go to the inside of the car and look at the pushrod on the brake pedal you can see a locknut on the rod. B4 making any adjustments push the pedal and see if you can feel the rod being a bit loose then contacting the master cylinder.
Also look for anything else that might be keeping the pedal from not returning to its stop, like a failed brake light switch or mis adjusted switch.
If you cant feel the rod contacting the MC then you might have to adjust the rod, but look for other things the prevent the pedal from returning to its stop first

Don't overlook the parking brake and that it can play a role here. It could be dragging.

Are both wheels locked?

to adjust the e brake remove the rear wheels then with the handle released adjust the rotor so its locked then remove 3 or 4 notches of the adjuster per wheel this should give the rotor freeplay and the handle should be adjusted to give about 3 or 4 clicks when its set ( note after adjusting the E brake , set the hand brake and release it,) check your work to see if the rotor is in fact released after you have tested the handle

Both wheels are locking up. I've do the adjustment to the brake pedal according to the WSM, and did not change the results. The only items I have not replaced are the rear calipers,which seems highly unlikely both would fail?! Could the abs unit cause the rears to lock, anybody?

can you feel the freeplay in the pedal, did you inspect the pedal for it fully returning when its released

ABS releases brakes, it doesn't apply them. Seems more likely the rear calipers or brake pads are sticking.

Pedal has some play but feels very tight.

Dan

Does the lock up occur when driving down the road? Does it happen when you apply the brakes, or does it seem to happen out of the blue when you're driving? What happens when you press the brake pedal when this lock up happens. How does the pedal feel? Is it soft like there's air in the lines, is it hard, or does it feel normal?

I sometimes have an issue where I'm driving down the road (60ish) usually on a highway, maintaining a constant speed, and I'll notice I have to push the gas pedal more and more to maintain my speed. One time it could only maintain speed in third gear. Lifting off the throttle and the car would slow down a lot. When I would press the brake pedal, it would be rock hard.

What I discovered was that something was making one of my rear calipers apply the brake on the driver's side rear wheel. It didn't apply enough pressure to lock up the wheel, but it did apply enough to almost make the engine overheat, and it burned off the brake pads on that wheel. I had to step on the brake to slow the car at one point, and as I said the pedal was rock hard. A few minutes later, I had to step on it again, and this time, it dropped to the floor. I pumped it a few times, and eventually got my brakes back. Everything returned to normal at that point.

It's happened a few times since then, and now I can recognize the signs of the problem and can catch it within a few seconds. The way I solve the problem is to stomp hard on the brakes (because I know the pedal will be hard anyway) and that usually causes it to behave again.

It's clearly a problem in the booster or it has to do with the pushrod adjustment. I've got a spare booster and maybe I'll change it out since I've got a spare that's collecting dust. It's time to flush the brakes anyway.

So if your problem has the same symptoms as mine (i.e. hard pedal when rear drags) give it a good solid kick and see if that resolves it.

I'm going to go have a look at my spare booster and see if it looks decent enough to install.

If you have your parking brake adjusted too tight, it will turn the drum portion of the rotor blue in a few minutes. Learned this the hard way as well. Unless you have parts missing that allow the parking brake to move around as it wants, I doubt that this is your problem. The parking brake will typically stay put unless you want to apply it. Even if it's adjusted too tight, if you can drive the car at all, the brake liner will eventually wear down and the resistance will decrease.