Anschutz 2001 Air Rifle Problem

[mnrok]

When pushing the cocking lever forward, (about the first 10%-15% of the movement forward not the actual cocking motion to the rear) the air is released with a popping sound and the pressure will not build up on the on the actual backward cocking stroke.
Chrony with 8.2gr pellets have fallen and erratic between 488.8 to 435.0.

A. Does this sound like seals or rings on the piston?????
B. I believe I have the skills to do the job.
C. Where can I order the seals and rings if this is the problem????

About what should the chrony read with a 8.2 gr pellet??

Thanks guys!!!

Walter