so, first look is the wiper there is it good shape if it old and foam could be gone and or a crumble mess, gun needs to come apart
now quick and easy just do the piston head O-ring and new wipe i have making wipes out a clothes dryer felt with a small drop of super glue it would last forever
but that is quick and dirty
the pellet probe is plastic and slides poorly but with lube as in white grease on the inside of the clam shells it can be improved
so, it is take, the whole thing apart but never the valve or something less
be aware that if you take the trigger group out the valve stays closed and if you pump the handle ever so little out pops the valve and then change that O-ring
the wiper is your key
no wiper no oil in cylinder no oil in cylinder bad compression no power and the chance of low power because a short compression stroke and or back leaking
confused yet LOL WIPER FIRST