I would like to add to Jorrell's post.

It is best to pull the vacuum from both sides of the system at once with a manifold set. Otherwise any air/moisture would have to be pulled through the metering device. The condensing side has smaller tubes and traps more goodies!

For your knowledge or any one else. I thought I would throw this out there....

The purpose of the vacuum is for several reason besides just removing the air and confirming a sealed system.

Air contains non-condensing gases and the big one...Moisture its a real enemy!

It is a major component to form a acid. Worse yet it can freeze inside the metering device during expansion and stop cooling until it thaws out. Creates intermittent cooling.

Moisture will boil off into a vapor more quickly at a lower temperature under a good vacuum. Moisture will most likely be in the condensing side because its molecule is larger and gets trap in the dryer. Warming a used, not wet desiccant dryer/condenser during vacuuming will drive off moisture towards the pump.

A dryer is rated on the amount of "drops of water it can retain". So you want to keep the system content low as possible.

On a car, 28-29 inches vacuum minimum during the times Jorrell mentioned. Rise of pressure can be a leak or from moisture. Rate of rise will be quicker under a leak condition.

The most moisture boils off below 29 inches. The last thousand microns.

Trying to hold below 29 inches and get into microns of the last inch towards 30 will not be obtainable due to the automotive rubber hose design.

A/C is considered high Temperature low side, a lower head pressure refrigerant used for the system. Especially on the cars.

On solid tube type and low temperature systems like refrigerators and freezers we use different refrigerants and manometers that read out in microns.

That is why I add a little heat to the condenser with a heat gun. You can wish for a nice hot day and let the sun shine on it. I have even ran the engine to warm the radiator to warm the condenser. That helps boil any water drops out.


Vacuuming is just as important as putting in the correct amount of refrigerant.

If you think about it. It's half of the finished job.

To begin a job. You need knowledge and then tools!

For what it was worth.

Phil