Louis, is the seal too tight a fit in the cylinder ? Also is it a thin lipped parachute seal design ? Regarding the hard cocking I have found a strange phenomena on a lot of old MK 111 rifles that the cocking lever is very tight in its hinge bracket, giving a lot of resistance. Disconnect it from the piston and see if it swivels easily. I have had to shave off some metal to make them move smoothly.

Baz