Joined: Jul 2015
Posts: 7,336
Likes: 3,199
From: NW Oregon
Bikes: 1982 Trek 930R Custom, '91 Diamondback Ascent w/ XT, XTR updates, Fuji Team Pro CF road flyer, Specialized Sirrus Gravel Convert, '09 Comencal Meta 5.5 XC, '02 Marin MBX500, '84 Gitane Criterium bike
have you tried putting some very light grease on the old o-ring yet?
clean the bore and piston, lightly grease the o-ring and bore, put the pump back together, and give it a test.