CRS760-140 Pump Rod Cup Seal (snap in style)
[CRS760-140]

I notice when holding my rifle pointed to the ceiling, and the forearm was extended in the up right position ready to pump, that it would slowly compress to the closed position. Therefore, air was being bypassed by the Pump Cup. This Pump Cup absolutely works fantastic on my 1984, Model 66A, Pumpmaster 66. Worked so well, I turned around and ordered another one right after, to have a spare.