1) Check the breech seal - they usually harden up with age and don't offer a very good seal. #### the gun, load a pellet, lay a tissue over the breech area, point the gun at your pellet trap, and fire it. If the tissue jumps, you need to either shim, or replace the breech seal.
2) Attempt to rejuvenate the piston seal as garmic advises, above. Another good oil to use is pure silicone oil, obtainable at the local hobby shop as radio control car shock oil.
3) Apply a few drops of at least 30 wt non-detergent oil through the cocking slot of the action to lubricate the piston, mainspring and spring guide.
If none of the above brings the gun back up to snuff, it will probably have to be opened up, and the course of action assessed from there.