Compressed air is not the best to use to purge the system. Compressed nitrogen or CO2 would be the best bet. as you know compressed air contains a large amount of moisture which can cause alot of trouble in the system. if you use R22 to put pressure in the system be shure to recover it and purge the system with nitrogen/CO2 again. if your car is inside out of the wind soap bubbles work well and are cheap.