If the pistol works in the same way as a Brno TAU 7, moving the cocking lever backwards towards the grip, dumps the CO2.
I believe you have to cock the pistol by moving the cocking lever forwards towards the muzzle before inserting a CO2 capsule to avoid losing a small amount of gas; and release the residual CO2 before changing the capsule by moving the cocking lever back.
Look here for further info;
http://www.pilkguns.com/tenp/default.htm
Hope this helps.
ATB
Mike.