omg another brake question

Yes, i am asking another brake question sorry!

The one piston in my rear brake caliper will not return, only when i take it apart and push the piston back in will it return :S

I bled the brakes last night (first time ever doing it, front took me about an hour and a lotta fluid, back took me 5min and much less fluid ) and i got them bang on, still getting all the power not spongy levers or anything.

Thanks everyone!

Jono