the first thing you need to to is draw the system down with a vacuum pump for about an hour or so. you do need a set of gauges as well so you can see when the maximum vacuum is reached, should be 30 inches.
once you have held they system at 30 inches of vacuum for the hour, minimum as i have seen some that needed more time, you can then tap the cans and fill the system. you want to see system pressures of about 40psi on the low side, and between 175-200psi on the high side with the compressor running. and ALWAYS ALWAYS ALWAYS fill the system through the low side.
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