Brake Gremlin '88 528

E28 technical advice asked and given! Troubleshooting, modifications and more.
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ColinH
Posts: 11
Joined: Feb 28, 2016 6:42 PM
Location: San Diego, California

Brake Gremlin '88 528

Post by ColinH »

Hello all,

My 1988 528e/i has an extremely odd habit of locking up the front brakes.
Originally (about a month and a half ago), the car would have an issue of stopping itself on really slow coasts, like in a parking lot. The car would stop as if I was softly touching the brakes. I assumed it was the pads just rubbing, which seems to be a somewhat common issue on e28s, and I imagined it was an issue that could be solved soon, but not immediately.
About a month ago, on the freeway, my front brakes locked up. I managed to get it over to the shoulder and tow it to a pretty well known mechanic who has always taken care of me, and more importantly, my car. Originally, we pulled the trigger on rebuilding the calipers and replacing the rotors after the damage. I got the car back and I locked up on the freeway home. I called my mechanic and he told me to disconnect the booster hose and tape off the vacuum to the manifold. the problem was fixed! But with no boost on an e28, driving back in rush hour traffic was a nightmare.
The booster was then replaced. I got it back, same issue. Then the master cylinder. I got the car back this weekend and it's back to close to the original issue. I pulled the ABS relays and I got about a 15 mile trip in before the pedal got stiff and the brakes started rubbing. My mechanic has found that if the problem happens again, I need to turn off the car, pump the brakes about 15-20 times, and the brakes will free enough to get me home- which is true. With the relays still pulled, I can get about 3/4 of mile on the car before needing to turn it off again. pedal feel is all over the place, but I find that the car will tell me through a stiffer pedal that my brakes are about to lock up again.

Car is going back to the mechanic again tomorrow but I would like to hear if anyone else has experienced this issue before. I noticed a similar post brought up a few days ago, but looking through RealOEM, it does not look like the '88 models have "the bomb."

Any and all help is greatly appreciated!
Federico
Posts: 385
Joined: Sep 16, 2013 6:38 PM
Location: Argentina / California

Re: Brake Gremlin '88 528

Post by Federico »

maybe... bad brake hoses?
You could test that by getting it to lock up, lift a wheel and crack the brake bleeder open. If it frees up, the hose is probably bad.
Mike W.
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Joined: Feb 12, 2006 1:00 PM
Location: Covid 19 Life during Wartime

Re: Brake Gremlin '88 528

Post by Mike W. »

M20 cars, which the 528e is, use vacuum boost, not hydraulic, so correct, there is no accumulator.

Bad master cylinders have been know to cause this, but more likely is it's out of adjustment. There should be some free play when you step on the pedal. Not a lot of course, but some, at least a quarter of an inch, maybe a half. But some. If there isn't, things can get weird with the brake booster trying to apply the brakes when your not, because it thinks you are. It's possible it's a bad booster, but I doubt it.
ColinH
Posts: 11
Joined: Feb 28, 2016 6:42 PM
Location: San Diego, California

Re: Brake Gremlin '88 528

Post by ColinH »

Thank you all who have input their comments,
Car has been at the mechanic for a day now.
No leads as to what the front brake problem is yet.
I have had a weird problem with the pedal as when I first start the car, the brake pedal feels "normal" and as I drive it, it seems to stiffen: eventually leading to a lockup.
Turning the motor off and pumping the brakes leads me to think there must be some kind of blockage in the way.
Maybe a bad valve on the ABS? Maybe some dirt inside the lines, but that would make one side of the front lock not both?
gadget73
Posts: 680
Joined: Nov 22, 2017 11:30 PM
Location: New Jersey

Re: Brake Gremlin '88 528

Post by gadget73 »

If its both fronts about equal, I'd agree its probably not a brake hose. I have had that on one side. I guess its not impossible you have 2 bad hoses but to have them both go at the same time in the same way seems unlikely.

Last time I had this kind of thing it was the master cyl pushrod adjustment. Replacement MC or booster had slightly different internal dimensions than what came off, and the rod wasn't allowing the master cylinder to fully return and let the pressure off. Heat from the engine expands the brake fluid and you slowly get more force on the calipers. That makes heat down at the brakes, which makes the fluid expand more, and it goes until the wheels lock up.
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