I think he is talking about the caliper not the piston. The only way they come out is when they are disassebled but this still should cause a problem since they are made to do this becuase of rebuilding purposes. The only problem that I can see and would have the same symptoms that your speaking of is if the boot around the piston was ripped. Did you have the piston out of the caliper when you the brake job? Or did you hit the rubber boot while in there?