The idea behind vacumme pumping a system is to remove as much moisture as possible and to virtually eliminate all non condensibles (like air) when you begin unscrewing or in the case of 134a systems Slide the release tube, you hear a his of air entering the lines. SOME of that WILL get into the system you just evacuated.
Before removing gauge manifold, I always pressurize the system. either with nitrogen to check for leak DOWN, or with refrigerant to charge the system.
why would you want to use such a small pressure amount to determine if there is a leak? 15 psi (approx 1 psi vacume and 14ish psi atmo) or 100 psi of pressure. which do you think is going to show a leak better???
also, if you are going to use the can to charge the system, the hose that hooks up to your manifold should thread onto the can tap valve. if not get a tap that will.
John