If it is not broken clear thru I would just put a little silicon gasket sealer on it. You might get away with this even if it is broken if it will go back together.As long as it can't leak air ir will be OK.Just use some marvel mystery oil or some air tool oil,etc, Transmission fluid works good.If you removed the sleeve/piston etc just make sure to get every thing back in right, with the bypass and exaust ports on the proper side etc,.