This is my oldest airgun and longest owned - fully original parts but time to time the gun becomes (hard to explain) but stiff to cock and sort of cocks in stages - almost like the piston ring is dragging. It's all properly lubed in the right places and I replaced the original ring (from Denis Hiller many moons ago) and when stripped the piston slides beautifully up and down, had it apart so many times.

IIRC I had another ring from Dennis and smoothed it down years back to help but don't want to ruin my last one.

Also IIRC adding a drop of oil or two helps but again causes dieseling so don't want to keep doing this.

Any ideas as it seems to put a strain on the elderly cocking linkages (which are also correct and working properly).