Okay so I took my rear tire off to change as usual the other day. After reinstalling the wheel setup, I reset my piston to put onto the rotor. Went to use the rear brakes and NOTHING. So I bled the brakes, the pedal firmed up, but then just got real "spongy" when I tested (but still fairly firm if that makes any sense). Tried bleeding again, same deal. Noticed the piston still moved, but seemed to not go any further (kinda a back and forth motion onto the pad). So I took the caliper back off, checked the piston while free and it came out as it should. So I put them back in, bled yet again, and still not strong enough to press the pads. What could be the issue? Should I just get a new caliper/master cylinder combo and be done with it? Thanks in advance.