Hi Mate, Its not hard enough ! IE short life ?
Measure Cylinder bore, Get out your PTFE, Machine to size, All dimensions ! ( Making the fit into the Pump arm head, Tight, ) Drop in boiling water & Fit, Job done, Good as new, Alternately, ? Machine new pump head, with Double O ring ( ALA Crossman Pistols " Modded ) Easy fix ! Cheers,