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.

I checked this morning @ 8:00 and the vacuum is holding, no movement on the gauge. The micron gauge reads 510 microns.
I think this is a good thing, correct?

The truck will probably sit for a week before I can get to the point where I will be able to start it. This should not be an issue, correct?


kenlou wrote: Sun Jul 20, 2025 6:21 am I checked this morning @ 8:00 and the vacuum is holding, no movement on the gauge. The micron gauge reads 510 microns.
I think this is a good thing, correct?

The truck will probably sit for a week before I can get to the point where I will be able to start it. This should not be an issue, correct?


Cuisser: 45/boil/45 is fine as well, imo. Just these 24 to 48 hr vacuum cycles on an auto a/c system seems a little over board to me.
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Ken, that sounds good to me.

kenlou wrote: Thu Jul 03, 2025 10:45 am
I drained the new compressor and added 4.5 ounces into it and spun it 10 times to get it circulated in the compressor…..CORRECT?

John replied:
That procedure is normally correct as long as the compressor has been installed (along with everything else), just before pressing the button, and the system is ready to run. The theory is to clear the chambers above the pistons of any oil so that the compressor doesn't "try" to compress a liquid, which can't be compressed, and break something internally, like a connecting rod. Discussions have it that both ways are acceptable without any consequences, although I prefer the "fully assembled" method.

Could someone explain what is involved in the “fully assembled” procedure to me?

Better yet, just to confirm, I will rotate the compressor clutch 10 times after it is installed in truck, correct?

Kenny

Ken: Thank you for the forum support as well as product support.

Here is my next plan of action: (the truck is a non running truck at the moment and the a/c system is empty)

I plan on puffing in some 134 on top of the vacuum, and use a leak sniffer to check for leaks.

With the two hand valves on the manifold closed and the micron level stabilized, I will remove the hose from the vacuum pump and connect it to the refrigerant can and open the cans valve pressurizing the center hose up to the manifold center tap.

I will then loosen the center hose at the manifold to bleed off the air in the hose to get just refrigerant coming out and tighten the hose.

With the refrigerant can inverted I will crack open both of the hand valves letting the refrigerant into the system as a liquid.

Once the pressure gets up to 25 psi I will then flip the can back over so its upright and let it continue to put more refrigerant into the system. This is to get the hose filled with vapor and not liquid.

Once the can is empty, I will close the hand valves and proceed to check for leaks with the sniffer.

Is this the way I should proceed?

I am using a digital weigh station to weigh the refrigerant.

If I find no leak, I will try to get as much of the 33 ounces the truck calls for into the system. Would that be ok to do it that way?

If I do find a leak I will make sure that no one is watching when I start my hissy fit.

I know that it is not advisable to leave a system in vacuum for an extended period of time. Would 3 weeks be too long to leave it sit in vacuum?

I also know that installing liquid 134 into both hi and low sides is only acceptable on an empty non-running system and that you NEVER add liquid 134 to the low side on a running system.

Basically, the way we did it in the shop. We had an R&R machine to reclaim the refrigerant if a leak was found.