I found out that what needs to be done sometimes is you need to use a
C-clamp to help push the piston back especially when it gets to be
almost fully retracted. Also, some brands of brake pads are too thick
so they must be sanded down (with an electric sander) until they fit.
It can be very frustrating trying to rebuild old calipers so anyone who
wants to do it better have patience and an alternate vehicle to get the
extra parts that you'll most likely need otherwise get new or
professionally rebuilt calipers.