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Vacuum vs Pressure Leaks

ewstan on Fri October 19, 2007 7:13 AM User is offline

Year: 1987
Make: Mercedes
Model: 300D
Engine Size: 3.0L
Refrigerant Type: 134
Country of Origin: United States


Is it possible for a system to hold vacuum but leak under pressure or vice-versa?


bohica2xo on Fri October 19, 2007 11:09 AM User is offline


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

GM Tech on Fri October 19, 2007 11:40 AM User is offline

Double lip shaft seals will leak under vacuum if the teflon is damaged- yet the rubber lip will completely seal under pressure- seen it many times if the shaft seal protector tool is not used and the teflon is knicked.......vacuum leaks can take time to show on your gage- don't be in a hurry to charge after a deep vacuum..........

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

ewstan on Fri October 19, 2007 12:01 PM User is offline

Thanks for the replies. Reason I ask is that I just replaced the evaporator and the system has been sitting under deep vacuum for about 12 hours with no visible gauge movement. But there is a slight UV dye stain behind the clutch.

I can't rule out that the dye came from previous work on the system. Two shops worked on it without finding the evaporator leak and left a fair amount of dye around the A/C components.

I'm reluctant to install a new compressor without being sure the seal is leaking.

Would a reasonable course of action be to charge the system and see what happens?


mk378 on Fri October 19, 2007 2:33 PM User is offline

Dye there is a pretty good sign your compressor is leaking. You might charge just a couple oz of 134a so you have positive pressure and see if it is the kind of leak that will hold vacuum but open up under pressure. Either wait a couple days and see if the pressure goes down or check with a sniffer leak detector if you have one.

ewstan on Fri October 19, 2007 4:56 PM User is offline

Thanks. Will do.


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