It's supposed to be a sealed system. If the pressure in your system is the same as the ambient pressure, it isn't sealed. In other words, you have a leak.

While there is a "high" side and a "low" side. It's all part of the same system, and it is supposed to be sealed. A leak on either side, will result in a leak that affects both sides.

Pulling a vacuum not only gets rid of air (which doesn't really work as a refrigerant) but also water (which can be detrimental to the functioning of the system.)

When you pull a vacuum, it is pulling a vacuum across the whole system (there isn't a difference between high and low sides.) In the best of worlds, you should pull a vacuum on the system and wait 24 hours to make sure it stays a vacuum. Only then do you add refrigerant to the system.

Of course, most leaks will show up a lot faster than that, so most mechanics won't wait 24 hours.

On Jul 4, 2011, at 5:36 PM, John Rodgers wrote:

> What does this vacuum check do for the system in terms of servicing
> the A/C system.
>
> When I put my A/C system back together next week, the pressure in the
> system will be ambient. Is the only purpose in pulling a vacuum check to
> enable the refrigerant to enter the system more easily? Or a check for
> leaks? And do both sides of the system et checked?