It does sound as if a piston is sticking.
Trouble is, if there was the remains of a ring of crud around one it may, depending on how far along the length of the piston it was, have caused some damage to the seals. Best bet is to pop them out and clean the pistons thoroughly, changing the seals at the same time would be wise. Use OEM ones.
The pistons do have a very good chrome plated surface so hopefully you'll find no rust pits anywhere that matters but, if there are, replace them too or they'll wreck the seals.