535iS Front Brakes Won't Release

The front brakes on a 1987 535iS will not fully release until the cars sits & cools down. This is not just a little drag, they are on to the point of seriously affecting acceleration. The passenger side rotor is very hot, driver side seems much less affected.
Problem does not occur right after a cold startup, it has to be driven a few miles. Get this...it also happened once just by running the car in the driveway until hot (it was a very hot day). Pads are new & not hanging up, also has new rubber brake hoses & fluid. This car has a hydraulic brake booster - could it somehow be self applying? At this point I'm not sure if the problem is with the master cylinder, the hydraulic brake booster or something else.

Charlie B.

Sounds like your caliper pistons are sticking to me, but remote armchair diagnosis & speculation via internet only goes so far. Me, I'd pull the calipers, remove the pistons, and take a look at the pistons and piston bores in the caliper. If going that far, I'd have caliper rebuild kits on hand (which basically are piston seals and dust boots), but if your pistons or piston bores in the caliper are rusty/corroded/pitted/etc. (which wouldn't be surprising after 25 years) then you'd need new calipers, as a new rubber seal on old-no-longer-able-to-function-properly parts won't resolve the problem.

If you had one front side sticking badly, you should be having to fight the steering, I'm thinking it would pull hard to whichever side is sticking. One side sticking I think would pretty much eliminate a problem with the booster circuit, since that ties into the master cylinder before the individual brake lines.

Search mye28.com and see what you find, and if nothing comes up, relay your issue, the gurus there I think may have experienced almost everything humanly possible with E28's.

This is not an unknown problem. Drive the car until it happens, then pour cold water onto the master cylinder to see if it releases the brakes. If it does, you need to replace the master.

There are more details and an explanation over in MyE28.com.

The cold water test worked! Thanks for the tip, I had been going round & round on where to begin.

Thanks for your suggestion. Right now its looking like master cylinder, so we'll see where that leads. The mye28 site looks to be a great source of help, thanks for that lead.

I believe the last time I changed one on a 535is I used a 750i master-cylinder. Worked well with my M5 brakes and cost less than the "correct" part.