Stumped on sticking front brakes

[Fabs]

Hey guys I've been banging my head against a wall trying to figure out what to do to fix my sticking front brakes on my turbo e30.

My front brakes lock up after driving for a while, usually fine on the highway but happens frequently in stop and go traffic. Also happens if I let the car just idle for a really long time. Unplugging the vaccum hose to the booster releases the brakes instantly. Brakes are very strong, I'd almost describe them as touchy and I almost always stop before I really feel a wall of back pressure on the pedal if that makes sense. When I'm at a stop and put pressure on the brakes feel firm.

Things I've tried:

-replaced the booster (with an unknown used one) but last booster I got originally in the car was slightly unbolted at the back to create an intentional vaccum leak I think to avoid the source of the problem

-bled the crap out of the master cylinder and all of the brake lines.

-adjusted the clevis on the back of the booster in to ensure there's a bit of slack

-car was missing the brake return spring (I've ordered one) but rigging up a bungee to pull the pedal back doesn't help so I don't think this is it

Girling brake booster and master cylinder.
turbo m20b25
Pulls 22inhg vaccum at idle, highest I see is 26inhg when reving the engine.

Things i am considering trying:
-New MC,
-New Booster,
-Simplifying the vaccum lines to the booster, currently there are 3 vaccum lines and 2 check valves that connect before reaching the booster... (1 before throttle body, and 2 after)
-Re-bleeding the system again?



If anyone has had this problem please help me, im super frustrated....

[packratbimmer]

I have been having a tough time with calipers sticking over the last few years. The pistons rust rapidly after a rebuild and jam. This year I'm going with new aftermarket pistons and hoping the plating is good. The chemicals they use on the roads are brutal. Use an infrared thermometer to read rim and hub temps after a drive. Currently, my right front wheel will nearly burn ones hand, but the left is cool after driving. Just waiting for the snow to melt.

[Fabs]

I have been having a tough time with calipers sticking over the last few years. The pistons rust rapidly after a rebuild and jam. This year I'm going with new aftermarket pistons and hoping the plating is good. The chemicals they use on the roads are brutal. Use an infrared thermometer to read rim and hub temps after a drive. Currently, my right front wheel will nearly burn ones hand, but the left is cool after driving. Just waiting for the snow to melt.

Hey there, thanks for the reply! I don't believe this to be the case, I've checked out the calipers and there doesn't seem to be any sort of rust around the pistons, pushing them back in with a vice feels smooth, and the pistons immediately release when the booster is disconnected

[packratbimmer]

Have you replaced the flexible hoses? I have heard of an obstructed hose effectively acting like a check valve and not letting fluid return and therefore keeping the calipers engaged.

[Fabs]

Yes all flexible hoses were replaced with braided lines. I believe I've sorted the issue, haven't had a chance to go on an extended drive yet.

I just replaced the master cylinder, but I believe my actual issue was the push pin inside of the brake booster not being fully anchored in and this was probably causing the mc to be unable to relieve pressure.

If anyone has this issue and stumbles across this in the future, check to make sure your booster push pin is centered and quite securely in place. My pin fell out when removing my MC. If it's not in place, then with the MC off have something or someone press the brake pedal In, and take a long socket about the same diameter as the ring that holds the push pin in place around the pin, and gently tap that ring and pin into place until it's centered and holds firmly. Be careful when re installing the master to not torque on this rod and dislodge it. I believe that's what I messed up when I was doing mine.

Hope this helps someone somewhere some day!
Thanks.

[TobyB]

if the booster's causing this, (and it seems you've proven this by dumping vacuum and replacing)

then the pedal pushrod is not fully retracting out of the booster. There are a few things that can cause this, from
corrosion on the pivots to a brake light switch that holds it down a bit, and you'll have to figure out the cause.

The fix is easy- get the pedal a little higher, or shorten the pushrod into the booster a bit. I honestly forget if it's
adjustable or not.

t