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vacuum vs filled pressure test for leak

deandec on Fri September 21, 2012 1:36 PM User is offlineView users profile

Year: 1995
Make: Country Coach
Model: Magna
Engine Size: 300
Refrigerant Type: 134a
Ambient Temp: 80
Pressure Low: 0
Pressure High: 140
Country of Origin: United States

I engaged an AC service to evacuate and recharge my 134a system.

After 24 hours, the pressure readings were not good.

I returned to the technician who said:

"a leak not found by the vacuum test could be found by pressurizing the system."

Not reaching novice stage yet, I need to ask:

Is this possible?

Is this probable?

ice-n-tropics on Sat September 22, 2012 3:12 PM User is offline

Here's a little known fact which partially answers your question:
Modern compressors use a double lip compressor shaft seal. The inner rubber lip seals against only internal pressure, not vacuum. The outer teflon compound seal seals against only vacuum, not internal positive pressure.
Therefore, if the outer teflon seal is damaged or full of wear particles (very common), vacuum test will fail. Repeated cycles of vacuum then positive pressure can suck wear particles under the lip of either, or both seals. Wear particles originate mostly from the outer seal which is 30% worn away by 100,000 miles.
If the inner rubber lip, only is defective, vacuum test will be OK, but positive internal pressure will leak past a defective inner rubber seal, as one answer to your question.
Caution: If a shaft seal installation protector is poorly machined or not used at all, The outer teflon seal will be damaged during seal installation and the A/C system will not hold vacuum.

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

Edited: Tue September 25, 2012 at 2:20 PM by ice-n-tropics

deandec on Sun September 23, 2012 12:13 PM User is offlineView users profile

Thank you for the explanation.

I have now exonerated that technician on two counts of doubt.

I am learning albeit slowly.

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