Vacuuming the system down is perhaps the most important step of the repair process.
Firstly the negative pressure you put the system under can be used to check for leaks and you clean out all the air , moisture etc from out of the system and being under negative pressure the moisture in the system will be boiled off as well .
Make sure you ensure all pipe hoses and connection on your tools are tight nothing worse than chasing leaks to find its your equipment.
With the manifold gauge connected to the system via the inline Schrader valve tap on the low side, turn the pump on and open the valve connecting vacuum pump to circuit. you should here the note of pump change and sometime you will see mist come out of the pumps breathing port as moisture it burned off.
You should vacuum down for at least 20 minutes to burn off moisture you should then shut the vacuum pump off and allow the system to hold its negative pressure for another 20 minutes.
The main gauge will be at minus 30 psi but it can be hard to spot leaks on this so it best to use a more powerful gauge, the vacuum pump I own has a very sensitive gauge and it will easy show up even the most minor of leaks so a always not and watch the reading of both.
If it holds for 20 minutes your fine to regas if not you need to find the leak.
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