For reference I'm working on a 1986 Corvette at about 5,000 feet above sea level (I'm in Colorado). The outside temperature is about 80 degrees, and humidity is relatively low.

The old R12 system had supposedly sprung a leak and the system had emptied, so I decided to convert it over to R-134. I flushed the evaporator, condenser, and all of the hoses separately, and installed a new compressor and orifice tube that came with the conversion kit I ordered. I also replaced a couple of the O-rings that were warped on the hoses. I now have everything hooked back up with the new R-134 valves on and a new accumulator in place, and am in the process of pulling a vacuum on the system before I recharge it.

The vacuum only gets down to about -21 in/Hg, and won't go any further even though I let it run for at least 2 hours. The thing is, when I close the valves on the gauge the vacuum will hold in the system at that point when I let it sit for an hour and a half, which leads me to believe there aren't any major leaks in the system.

What are some possible reasons the vacuum wouldn't pull close to -30 in/HG like it should, and is absolutely necessary the vacuum gets this low to remove all the moisture?

  • Welcome. My next move would to test my pump. Do you usually see -29 in/hg at that elevation. We get that low here but we are at about 50 ft above sea level. Commented Jun 9, 2016 at 18:03
  • Check the pump first, I have the Robinson type and it "uses" oil for some reason, and it won't pull fully. You should be able to pull lower than that even with small leaks. The "leak" question is after you shut the valve and it still shows the same vacuum 30 minutes later. I suspect the pump.
    – SteveRacer
    Commented Jun 9, 2016 at 22:22
  • I am at 6000ft and can only get down to -22. I'm using a vacuum pump and gauge manifold set that I bought at harbor freight. Not sure if they are accurate but the AC worked fine afterwards.
    – Don
    Commented Aug 23, 2019 at 1:02

2 Answers 2


I don't know the devices used for this job, but due to the negative sign, the pressure seems to be relative, i.e. difference to ambient pressure. (When the device is disconnected and doesn't run, the gauge should show a pressure of zero, too)

The absolute air pressure at altitude is shown in this plot:

enter image description here

(Sorry for the odd values, but I'm used to SI units...)

30inHg is the absolute air pressure at sealevel at standard conditions.
At your altitude, the absolute pressure is 25inHg.
(Sanity check: Denver Airport (1656m) currently has 832hPa, which fits really good into the curve.)

So you can't pull 30inHg, since then you would have a negative amount of gas molecules inside the system. The highest value you can get is 25inHg.

You get 21inHg which corresponds to an altitude of about 11,000ft, so this can't be explained by you being at not exactly 5,000ft.

Since the system holds the vacuum, I'd say there's a problem with the device. Either the pump itself or the pipes between pump and valve has a small leak. The pump may be defect, or the oil also used as sealant is low, old or contaminated. (Though it looks and sometimes smells like motor oil, it isn't, and using the wrong oil can heavily affect the performance)

Creating a vacuum is not only done to remove water (seriously, how much water could be in there, plus there's a dry cartridge), it's more because it's important to have no other gases except the R-134 inside system.
From that point of view, if you can't create a good vacuum, it may be possible to flush the system by adding a little R-134 and removing it again a few times.

(I really wonder why the pressure is measured relative to ambient pressure, since this leads to exactly the problem occurring here, too: Not knowing what max. value should be achieved at your location...)

Edit: Since people seems to have interest in this graph, here's the formula you can use for excel etc.:

Pressure = 1013hPa * exp (- altitude / 8400m)

1013hPa is the normalized air pressure at sea level in hecto-pascal. You can replace it by 760 Torr, 1.013bar, 14.7psi, 29.9inHg or whatever the pressure is in your favorite unit.

8400m is a constant in meters, you can replace it by 27559ft or whatever to match your unit of altitude.

  • The pump I used I rented from Autozone, and it looked to be unused as it was still in its original plastic. Unfortunately no other auto stores near me rent out these vacuum pumps, and I don't really want to shell out and buy one, so I can't really try for a different pump. However I did flush any gas left in the system by adding and removing refrigerant. The only issue now is the new compressor I received turned out to be defective. Commented Jun 12, 2016 at 1:44

Probably a small leak at a hose fitting or something similar. I think the easiest way to check would be to fill with nitrogen and check for leaks with soapy water sprayed on all joints and connections.

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