T9

my brothers t9 has acted up every time we have went to paintball. i helped him buy it from a friend of mine for 200. ok when he goes to use it as a sidearm he puts the 12 gram in and it leaks he tightens it just a little it leaks he tightens it all the way ... it leaks it leaks no matter what. it shoots fine if he gerts the 12 gram in without leaking. i thought maybe its the 12 grams so i bought a remote line for it. we went out to the woods with a coupkle of friends on the 4th of july. i hooked up his remote line and put a 20 oz co2 tank on and it leaked a lil bit until i put the clip magazine.. into the gun. the gun shoots about twice and it stops feeding the co2 or something the balls dont go as far and then it just stops i can see the powertube or something like that in the gun through a little hole and on the first shot it goes up and back and on the second shot the same but after that it slows down and it looks like its not wanting to slide back but it does slowly and when i twist my hpa tank onto the remote line the gun works just fine anybody know whats wrong or have any ideas? thanks for the help