If the air conditioning circuit has been opened and atmospheric air has entered it, it must be pumped out using a vacuum pump. If the system has been open for several days, the receiver/dryer needs to be replaced.
Start the pump and open both valves. Leave the pump running for 15 minutes, then close the valves and turn off the pump. The low pressure gauge should show more than 700 mm Hg. and this pressure must be maintained with the valves closed.
Warning! If the low pressure does not reach more than 700 mm Hg. within 15 minutes, there is a leak in the air conditioning circuit.
If there are no leaks in the air conditioning system, open the valves and continue pumping for the next 15 minutes, then close both valves and stop the pump.