As a follow-up to this, the pressure adjustment knob has to be open just about all the way when turning the gas on. Otherwise the sudden pressure will blow the relief valve. This knob is the big knob on the front (before you say anything, hey - we're better than that!).
The key is to open it to just before it comes off, then open the tank. When the flow starts, gently turn the pressure adjustment knob to where it doesn't leak anymore, and then perhpas another quarter turn or two.
The MA 957 is great, but I really wish that this critical step in setting it up would be emphasized more in the instructions. It seems like common knowledge now, but anyone new to using CO2 tanks has no idea.