Automotive Air Conditioning Information Forum

I think it is overkill to vacuum a system overnight. For many years, I've read the repeated claim to do this, which I personally believe some have simply repeated, because they read it on the internet. Pull a vacuum for 30 minutes, close the gauges, shut off the pump, and let the system boil off for 30 minutes. Open the gauges, pull another vacuum for 30 minutes, and charge the system. That's what I would do and did for many years in our shop when I was younger.

I thought I had heard that vacuuming overnight would ensure a really dry system.

I did what you suggested and it will sit overnight in a vacuum. That won’t be a problem, will it?

Kenny

kenlou wrote: Sat Jul 19, 2025 8:23 pm I thought I had heard that vacuuming overnight would ensure a really dry system.

I did what you suggested and it will sit overnight in a vacuum. That won’t be a problem, will it?

There are several reasons to pull a good vacuum. These include removing air from the system, and moisture will boil at ambient temperature at such vacuum.

I use 45 minutes to pull good vacuum, then I do a vacuum test for like 20 minutes. If vacuum holds, then I begin to charge the system. Obviously, I don't have a specialized automatic machine to do this....

If you don't have a leak, sitting overnight in a vacuum should not affect anything. You can pull vacuum overnight - it's your vehicle - but that's overkill.