Look inside your ASA (the thing that the tank screw onto) see that pin? Remember it.
Now look at your tank. See the small pin there too? When you screw your tank on those pins hit each other, which pushes down on your tank pin and opens the valve.
Some Piranha pins are too long, and the tank valve opens too fast, not allowing the oring to seal. Theres a few ways to fix this,
1. Sand down the pin inside your asa (the one I told you to remember)
2. Call up PMI and ask them to ship you a replacement
3. Sand down the pin on your tank a bit
4. Buy a new ASA, they're about 10 bucks.