only a couple of things can cause that, however, FIRST ADN FOREMOST: GET A NEW TANK OF AIR...once youve done that...
1) recock gun and try
2) replace cup seal and try it again
3) replace valve orings and try again
4) make sure valve spring is strong enough to close valve
it is probably the valve body mine is doing the same thing I contacted BE Service and it gave me this 1800#-1-877-376-7966 the valve costs 9.95w/shipping
you can check it by taking your gun apart and see if the pin seals flat on the body like it is suposed to