You can add three cans to an empty system without turning the key. I hate idling the engine & revving it up for so long trying to empty a can/can's. Remembered reading this tip in an A/C manual some time back & it works. Of course the system needs to be holding evacuation vacuum if it's empty, but this also works for topping off. All you have to do is have the cans in hot water & the freon pushs from the can. You can hear it going into the dryer. If the water cools just refresh & keep the water hot for it to continue to flow. I've had to do 3 A/C fills in the last couple weeks due to repairing an inop system full of leaks & corroded lines in a 780 I bought last month. Each time it has pulled 3 cans & then cooled great until I found another leak. Think I may have it repaired this time. Has held for 3 whole days this time. Now need to get the Aux fan working.