Brake emergency

I was thinking the same thing.

I had one in the shop that did the very same thing, on a front disc. A hard stab, and it wouldnt release. A second softer,slower application, and it reverted to normal.
Its hard to imagine this on the rear though where there are two brake cylinders on the same flex-hose.

Just curious, did you polish off the rust on the shoe,bearing pads?