Have you checked the pressures discharge and suction? You may have a leak and a system full of air. you said 'had it vacuumed'. I recommend holding the vacuum for half an hour pumping all the time. Then block it in and confirm that it is tight no decrease in the vacuum for another 30 min. Then charge liquid in th ehigh pressure side(engine off) until either the correct weight had been added or the pressure balances the can. Then start the engine and continue adding VAPOR to the suction side.