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Hey Guys, have a 94 R100R with front brake drag. I completely drained and filled with fresh fluid, but cant stop the dragging brakes. Anyone know what could cause this. Master cylinder rebuild needed? By the way, the bike has the 4 puck caliper with dual floating disk. Thanks

If the reservoir is venting properly and not overfilled.

It could be:

Master cylinder or caliper needs to have seals replaced.

Bulging old rubber/OEM brake lines?

Kurt in S.A.

If the dragging wheel does not rotate freely with the bleeders open, the calipers need attention.

If the dragging wheel rotates freely after opening the bleeders, but drags again after a few brake applications, the small return pinhole in the master cylinder is blocked.
This hole lets the fluid back into the master cylinder when the brake is released so no pressure remains in the line(s).
You can access it to unblock it (with a needle or tiny drill bit) after removing the fluid container but total disassembly is recommended.
(post edited for clarity)

One point,

When you refer to 'dragging', what exactly do you mean... It is quite normal for disc pads to rub lightly against the disc with the brake off. There's no positive action to pull the pads away from the discs. This is, in fact coinsidered to be a good thing because it keeps the discs warm and free of moisture which would affect immediate braking efficiency. If, on the other hand, there's any real retardation of the wheel, then, yes, you have a problem

Rob

The distortion of the square-oring seal between the piston and the caliper housing is what pulls the piston back after brake application. To a smaller degree, the eccentricities of the disk might "slap" at the pads to push the stack farther into the caliper.

Kurt in S.A.

I had the same problem with my Mystic.
I pulled the calipers, popped out the pistons.
Cleaned everything. Buffed the pistons and cylinder bores.
Reassembled.
Update:
Even though the fiche show no repair parts for the Mystic/R100R, the calipers are the same part as used on the R1100RS, which does show repair kits. So if you want new pistons and seals, they are available. $131 for the kit per caliper (4 pistons and seals), but better than the $747 for a new caliper (as per the MaxBMW website).

Kurt in S.A. wrote: ↑Tue Apr 13, 2021 6:36 am The distortion of the square-oring seal between the piston and the caliper housing is what pulls the piston back after brake application. To a smaller degree, the eccentricities of the disk might "slap" at the pads to push the stack farther into the caliper.

Kurt in S.A.

That is exactly what should happen but only if the pistons are free of corrosion and lubricated. I assemble caliper pistons using red rubber grease and that keeps them moving freely. If I went out to the garage now and spun the front wheel by hand it would keep going for over a minute only being slowed by the bearings and seals.

Most, if not all, of the (BMW) bikes I've seen towed in with seriously dragging/seized/smoking brakes over the years all had a solidly blocked return hole in the master cylinder. It seems very common on BMW's that never had a proper brake fluid flush their whole life.
I've seen wheels locked solidly enough because of this blockage that the bike could not even be pushed, the tire would simply skid.

In comparison, most of the cars I've seen exhibiting similar symptoms usually only had seized caliper pistons and/or guides.

Thanks to all!!! I have to say you guys are really good. Lots of great info!! I'm thinking maybe the return hole is easy enough to look into first. Where exactly is it located in the late model master cylinder? I do have the original 27 year old hose as well.
I did pump out all of the old fluid and replaced it with fresh and bleed. It is a bit better, but not turning free enough(not a light rub,or scuffing)
Hi Seth! Been a while! I guess failing with the easy stuff, ill bite the bullet and do the caliper rebuild...