[GMC Canuck]

When you first start to charge the system you don't need to have the compressor running. The vacuum will suck most of the refrigerant into the system. Once the pressure in the system and the tank you're filling from equalize , then you engage the compressor and slowly top up until the system is fully charged.

When you're getting your system charged, make sure they pull a good vacuum. The longer it stays on the vacuum pump the better. You're not just pulling the air out of the system, you're pulling any moisture out as well. Compressor oil is hygroscopic (takes on or absorbs moisture) so you're pulling moisture out of the oil as well. Moisture in the system combined with compressor oil creates an acid which over time is detrimental to the system. So it's best to keep compressor (I would say all, but mostly compressor) fittings capped until you're ready to make a connection.