I think i resolved the vacuum pump check valve issue. Took it off and cleaned it with WD 40. Seems to hold now. However, I think effectively testing the valve's action is rather difficult. Wouldn't be surprised if my testing apparatus was leaking. (Had to switch to a bigger conical fitting to shove into the metal vac hard line)

Is this check valve supposed to be sealed to the pump with a copper or alu crush gasket?? The valve has a circular ridge for one, just like a drain plug...