Don't understand what you mean by "don't charge under a vacuum." Correct way to charge an a/c system is to hook up guages, pull vacuum, close valves and switch vacuum pump for charging cannister(s), bleed air out of main hose, then open valves and charge. WITH THE ENGINE OFF I normally open the high side (red) valve - the vacuum will draw in the entire can. Close valves, start engine & a/c, then charge next can(s) through the low side only.

- JimY