Bad oring inside the tank more than likely.
There is a post on here somewhere by Madmax about this issue on random tanks. If you go to www.airsoft-innovations.com
then click on support and read the gun gas kit manual it tells you about this and how to check.
Saskatchewan Age Verifier! Contact for more Info.
http://airsoftcanada.com/showthread.php?t=174161 - Honour & Respect your fellow players! #Airsofthonourandrespect