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