Well, that black end cap holds the spacers and thus the o-rings in place. My guess (and it is only a guess) would be that if the air is coming out there then that would indicate a problem with the o-ring closest to that spacer - and by extension possibly with the spacers themselves. I do know that the spacers are wear items, and that after a long time they can need replacement. Any chance that yours is worn in a way that is causing it to put pressure on the o-ring in a way that is breaking the seal to the cylinder or piston? That is what I would look for first.
For the F10,I have nearly 12 rebuild kits, one has the upper cylinder o rings missingTwo belts one newNew Insert Ferrellkit for the lower cylinder The tower with belt drive and intact lower cylinderPrefilter set up with a McMaster car filter.Bunch of misc parts.Regards,Roachcreek