I'm looking to do some AC work on some of my vehicles and I've purchased a recovery system along with a 30# tank (empty), vacuum pump and a scale. Looking at videos on how this works, during the recharge, the car/truck is run with the AC on, which is supposed to pull in the last remaining refrigerant from the tank. Every time I've used manifold gauges and looked at low & high side pressures, they are always positive. Even with the tank contents going into the low side, there's positive pressure, so how will the tank ever become empty?
Setup like this:
https://youtu.be/O8s5SzUXtiQ?t=1165