Erm. I think there's a small breakdown in what we're referring to here.
The p38 uses floating calipers, i.e. only a piston on one side.
See in the below pic, the two rubber boots. These are the pins we're referring to. They might seize up and cause things to stick. (as well as the piston as you already know).
As asked above, has one pad worn more than the other ?
We might be totally wrong about this being the problem just giving you options for known issues
The front brakes use the same system.
G