AC Holds Pressure but not Vacuum

krstahl on Mon May 13, 2013 2:21 PM

Year: 1996
Make: Chevy
Model: Silverado
Engine Size: 5.7L
Refrigerant Type: R134a
Ambient Temp: 70F
Country of Origin: United States

Delphi compressor was OEM, replaced with rebuilt from O'Reilly.
New accumulator
Used, but tested and cleaned, condenser
Flushed all lines

I had a leaking compressor so bought a rebuilt, added PAG 150, and reused the rear pressure cutout from the old compressor. I flushed all the lines and hoses until clean. I also bought a new accumulator and replaced the damaged condenser. I installed a new orifice filter, as well. The new o-rings that came with the compressor were chintzy and did not hold during a positive pressure test so I inspected (they are pliable, no cracks, no nicks no cuts), lubricated (silicone grease), and reused the old o-rings. I pressured the system to 45, 50, and 60 psig over a period of two days. I performed a soap bubble test on all the fittings during each test and confirmed there are no leaks under positive pressure. The pressure gauge held steady in all three tests for over two hours. After each positive pressure test, I pulled a vacuum on the system to -25.5 psig. All three vacuum tests bled off within five to ten minutes. I have confirmed that it is not a leaking test valve, nor a leaking vacuum hose. I am out of guesses. Does any one have an idea what I should do next (other than taking it to a shop or junkyard) to solve this puzzle? I'll take it to an ac shop if I have to but would really like to do this myself if I can.

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Retired and DIY.

Edited: Mon May 13, 2013 at 3:28 PM by krstahl

Dougflas on Mon May 13, 2013 8:58 PM

Do not reuse any o rings, period. Do not use silicone greae on o rings or fittings. Use refrigeration oil or Nylog on o rings. If it holds pressure but not vacuum, I'd be looking at the compressor seal. What are you using to pressurize the system? If freon, then it could be leaking under pressure but you may not see it because of the temperature /pressure relationship. 25in vacuum is not enough. You may have a leaking manifold guage set or hoses.

krstahl on Mon May 13, 2013 9:26 PM

Thanks for the input. The reason I replaced the compressor in the first place was because another forum suggested that seals on older Delphi comps hold pressure but not vac, i.e. worn seals leak into the comp but design keeps pressure in the system. Even though I bought a new (rebuilt) comp I was wondering if that might still be the culprit. Maybe I'll have to spring for the extra $$ to buy new rather than rebuilt. I couldn't get to the dealer for new o-rings so thought that would be worth a try. I used the silicone grease because that is what is recommended for scuba gear (tanks and regulators, as well as on my underwater camera o-rings). I understand the comment to never reuse o-rings, but have had to do so many times in the field on other gear and fittings, always successfully. As a former materials inspector I deemed the existing o-rings suitable for use, they held the pressure whereas the set that came with comp wouldn't even pass a bubble test. However, new o-rings would be the least expensive means to start over on de-bugging this prob.

I pressurized the system with a small air compressor. I know that some might say I am introducing water into the system but the relative humidity in my area was at 9% today so I'm not as concerned about introducing water vapor here as I would be if I lived in the deep south or east coast.

I checked the hoses by pulling a vacuum on the ac and unhooking the valve from the comp for 1/2 hour. Then, I reinstalled the valve to the comp and the vac had zeroed out. However, as I write this, I could easily test the hoses/gauges by pulling a vac on them with the valves closed, then see what happens. That would be easy enough to do, if vac holds, hoses are good, if it leaks off, then buy new ones.

If others reply with other ideas I can hopefully apply them, as well. For now my next steps will be: 1) test hoses/valves, 2) new o-ring sets, 3) new compressor - ouch!

Again, thanks for the help and ideas. I won't be able to get back to the truck ac for another week - vacation calls!

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Retired and DIY.

WyrTwister on Tue May 14, 2013 6:38 AM

Replace both schrader valves , if you can . Lube them with refrigeration oil . If it works , it is a cheap fix .

God bless
Wyr

krstahl on Tue May 28, 2013 9:15 PM

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Retired and DIY.

krstahl on Tue May 28, 2013 9:33 PM

Sorry for the blank post above - don't know how to delete it.

Thanks to those who posted top help me. An update to what I finally did.

I replaced the condenser because I had hit a pheasant several years ago. I don't think it was leaking, but for $45 used, cleaned, & tested it was worth it. I returned the re-man compressor I bought and purchased a new in the box compressor for about $50 more. I also replaced the accumulator and the condenser filter. Finally, in the process, I messed up the upper (high pressure) hose fittings so I had to buy a new hp hose. That came with new schrader valves. All parts came with new o-rings, which I used. I cleaned all fittings that I had previously used silicone lube on, then installed the new o-rings lightly lubed with refrig oil. After all this I still could not hold full vacuum but it was very much better than before. I tested the hose fittings and fill valves and they held vac perfectly. So, there is still a small leak in my system somewhere. It could be under the dash board but I'm not going to tear into that.

I pulled vac on the system and installed the specified ~30 ounces of R134a with "sealant" in it. The charged vent temp was between 39F and 42F when running at 1500 rpm. Hopefully, the sealant will help close up whatever leak there might be. If not, I'll be recharging the system every year with another $45 worth of R134a.

Anyway, I had a lot of fun learning the AC system in my truck and I would tackle something like this again. It was just a bit frustrating that I did not have a 100% success. Still, it was worth it for the learning, fun, and the truck is now much cooler.

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Retired and DIY.

WyrTwister on Tue May 28, 2013 11:39 PM

Let us know how it does after a few weeks ? I am wondering if it is the seal , if it will wear-in and not leak ?

I agree , I would defiantly buy the new compressor for $ 50 more than the reman compressor .

God bless
Wyr