that appears to be an 03-04 prostock. fairly simple to replace. buy a post 2k pump kit Located here
and put it in place off the front block and pneumatics. simply unscrew the big screw that holds the front block on, take of the pneus and screw the pump kit in its place. i usually wrap a layer of teflon tape around the guide rod threads before screwing it in. to pull the trigger rod out there should be a set screw just behind the 3way, you can unthread it and pull the rod out. to replace the guide rod (rod from pneus to back block) just unscrew the back block adn screw the back block onto the pump rod that comes with the pump kit.