I think if it were me, I'd pull the air filter and block the pop-off closed, then retest. Is the check valve located after the air filter a requirement for proper pump operation? If so, is it working? I'm thinking in terms of something restricting the incoming air flow, or allowing leakage back through the filter.
Definitely check your amperage draw, but it just sounds like a low pressure problem to me. What kind of shape is the pump in?
