You can't go by system pressure with the engine not running, and it can be misleading with the AC system running. Part of what's in the system will be vapor, and part of it will be liquid because it's held that way by the pressure. If, for example, you start with 100 pounds of pressure, then bleed off some vapor, either due to a leak or you do it on purpose, the pressure would go down, and just like with water with the pressure removed, some of the liquid will boil and turn to vapor. When any liquid turns to vapor, it expands hundreds of times, and that causes a huge corresponding increase in pressure. You'll still have 100 pounds in this sad story.
As long as there is still some liquid in the system, if you remove any vapor, the liquid will vaporize, expand, and cause the pressure to go right back to where it was. You will not see a drop in pressure until all the liquid has turned to a vapor.
This is also what makes it impossible to know for sure how much refrigerant is in the system. There's only two ways to know. One is to recover everything, then inject a known amount. The other is to use a sight glass. There's only two manufacturers that I'm aware of that use sight glasses. That is only effective on Chrysler products. You simply keep adding refrigerant until the vapor bubbles no longer appear in the glass. Ford started using that too, but there's a problem with their system. Even when the system is full and properly charged, there will still be bubbles in the sight glass. Over-charging an AC system can lead to compressor damage, so that sight glass really is not a good tool on Fords.
Thursday, September 3rd, 2015 AT 4:23 PM