The pumps seal is bad so take it apart and cut a piece of leather out to fit
Coat the leather with Vaseline and it will operate just fine for you

Hello. Go to this site and down load the manual and the parts diagram for your pistol http://www.crosman.com/airguns/service/manuals/crosman-pistols

This info will give you knowledge to identify the faulty part. Hopefully they still have it available. J.