"Locked" Ultegra brakes
 

Yesterday, I installed new rubber pads on my rear brakes, I took the shoes out and apart to do this, nice and easy until I tried to put them back on. The whole "horse shoe" shape brake locked, so to speak. I tried
to unscrew the whole thing, loosen them up, but clamped itself in real tight, and it won't budge. Both shoes are pressing with tremendous against the wheel rim.
What gives? How do I get them loose to adjust the whole brake?
thanks,
Corsaire

Release the parking brake.

Did you release the cable from the brake assembly. If not that would be the first thing to do. also make sure that all the cable housing is properly seated in the stops and didn't get hung up on the outside of a stop while you were working on the pads. How worn were the original pads? If they were really worn you will need to release the cable to allow for the new pad width.

OP: is your bike in a workstand? If so, follow the rear brake cable routing to make sure the cable isn't being clamped against the frame where the bike is clamped into the workstand. This sometimes happens and the cause isn't immediately apparent.

I can't think of anything beyond what's now been suggested that would cause them to lock up like that. For experimentation purposes, you could release the cam lever on the brake caliper; that would loosen the brake somewhat. You might then be able to get the wheel out so you can have easier access to the brake caliper.

Good luck!