That odd cam-over deal at the top looks like it would be more efficient breaking a vacuum, than relieving pressure (innie vs outie)... is there a nipple on the other side of the canister one might hook a vacuum pump to? Although I believe that a port in that spot at the inlet to the impeller would create a powerful vacuum (and air leak) on it's own, and that you should never see pressure on that side of the pump. I can't see how it would get past the impeller to get to the pressure side either.
Any Berkeley experts out there that can tell us if that is pressure, or suction right there where his line is hooked?