Have an older Crossman Powermaster 760. Has the wood stock and handguards, blued steel barrel and leaf sight. Not certain of the age, but it has a square button that you push in to load and unload bb's. I'd guess either late 60's or early 70's.
Issue is yesterday a 177 pellet was put into the breach while lead BB's were inside the rifle. First pellet shot worked, but on the second shot the pellet jammed in the barrel. Managed to use a coat hanger to clear the bore.
Now when you pump the rifle it doesn't hold air. Before there was tension when pumping the rifle, now there is none. Tried to oil the seals, but so far it makes no difference.
Congratulations Dave, you've made it 40 years on those old seals, and it's finally time to replace them.
That old girl is loosely referred to as a Phase 0 gun, made between 1966 and before July 1975. It'll need a 760SC Seal Kit that you might still be able to get from Crosman. The kit contains part numbers:
111A026, 130-034, 130-035, 140-058, 760-037, 760-042, 130-057, and 1400A029.
Give them a call and see if they can fix you up with the parts. Good luck!