I am sure you know this - but JUST in case.
If the S5 body get's over travel when being cocked (pneumatically is less likely than in pump configuration) - you can get a mushrooming of the back of the slot in the bottom tube.
However, in this case it sounds like you had a burr in the allen key access hole (to get to the lug) and it was dragging here.
Odd - but I am glad you got it sorted.
Try a cleaning and some Dow 55 - but I THINK those are stock o-rings. I have some in my kit if you need me to measure.
Last edited by Talfuchre; 04-01-2013 at 10:34 AM.