Disk brake locked up

After topping off my brake fluid and pumping brakes, the brake is locked up. Can I just open the valve to let off a little pressure or should I bleed them and replace the fluid? Any thoughts on the cause? Thanks!

You might need new seals on the pistons or a master cylinder rebuild.

Did it work well before?

They had gotten a little soft but seemed to be okay prior, then I pumped them repeatedly and now there is almost no play in the lever.

If you bleed them and the work fine for the moment, they could lock up on the road and you would get acquainted with the pavement real fast. I would rebuild the MC first and if that doesn't fix the problem i would rebuild the caliper, brakes is somthing you don't want to take chances with IMHO

Maybe you've overfilled the reservoir and there is no air gap at the top to allow fluid back up?

May have gotten some crud in the bleed hole inside the master cylinder. Try sticking a small needle through the holes the clean out any blockage.

Thanks for all the suggestions. Took the master cylinder off and it was full of crap. The parts guy at Bob's BMW said that 60-70% of the time the rebuild kit won't work due to too much pitting, so I ordered a new one. Hope he wasn't just giving me the up sell. I've never had a problem with Bob's, anybody out there have a source for parts that they like better?

Talking about rebuilding brakes i.just rebuilt the disk brakes on my RD (original setup).

The really important part of rebuilding them was to remove all the gook that sits in the ridge in which the big round seal of the caliper sits.

After many many miles the dirt that builds up in there will 'push out' the seal,not letting the piston slide.in and out properly.
My local brake expert told me that good brakes should be able to push the piston in with just one finger....

I used a really really soft polishing dremel tool at low rpm with brake liquid to get rid of the goo.

Do not scratch or pit anything in rhe area where the piston is sliding....