RF brake locked up. Why?

There has to be roiling. Stick your finger over the C-port, and feel the returning brake fluid.

To test you booster;
engine off, M/C lid on and clamped; pump the brake pedal to deplete the vacuum, about 5 or 6 times. On the last time, keep the pedal down and pushing moderately. Then start the engine. The pedal should drop down almost immediately as the vacuum chamber evacuates. How far depends on how hard you are pushing; I'll guess;more than one inch, less than about half way to the floor.
The first inch is taking up slack and closing the C-port.
You need the bottom half for emergency use, if one end (front or rear) catastrophically fails. When that happens, the pedal drops nearly to the floor, but, if the adjusters are correctly adjusted, you should still have enough pedal-travel left with which to stop the car.

If your pedal is hard within the first inch, you better check to make sure your C-port is still open! You can shine a light in there and maybe catch a reflection off it, but I just stick a wire in there, which should penetrate to the bottom of the bore which could be an inch plus the casting thickness. But the roiling is the proof that the fluid is returning.
If the piston inside the M/C is not parking all the way back, something is wrong.