I have a Tau 7 with a similar issue, the o-ring in the piercing cap blows and looses all C02....after the first shot! Fitted new 0-rings direct from Sureshot, cocked the pistol to take any pressure off the hammer, but the same thing happens. This is not a case of a cold piercing cap/seal, it even happens on first use. It is an unusual design, having an o-ring slide inside the walls of the chamber (as opposed to the end of the chamber) with the expectation that it will seal successfully every time.
Might try alternate Urethane o-rings of a suitable size; I suspect a more rigid 0-ring might work better and also fill the groove better in the piercing cap.