Last fall I noticed a small hiss when starting the a/c. I looked around the system and noticed a thin film of oil/dirt around the high valve. I guessed my high pressure shraeder valve to be leaking so I stopped using the a/c as I had read that an empty system can attract moisture and really bad/expensive things will follow.

I finally tackled this problem today. I replaced both valve cores using a device that keeps pressure in the system. As far as I was able to tell, no coolant was lost during this removal. After attaching the manifold gauges to the system, I discovered my psi on the high side is around 37 (car off, temperature around 40F, humidity 50%). When I tap the valve with a screw driver, a hiss can be heard so I am assuming my system is still under pressure, even if it is low.


1) I’ve read that if a system is empty, one needs to apply a vacuum to eliminate any moisture. Moreover, if the system was empty for a while, one should replace the receiver/dryer. Does this advice also apply if the system is low but not empty?

2) I’ve read that system leaks on the high side don’t leak much oil. Is that true?

3) I've read not to charge a system below 55F. Is this because of the difficulty in estimating the correct amount to add? Would it be fine to add just a little to move my system from low to medium and raise it to 100% once the temperature rises above 55 degrees.

  • 1. No 2. No 3. Yes
    – Moab
    Feb 8, 2020 at 21:14
  • The manual will give the pressure requirements . I would not fix it unless I knew there was a problem. I did refill a system once ;1 evacuate,2 flush with one can, 3 evacuate, 4 fill . No problem , no new dryer. Nov 1, 2021 at 23:53

2 Answers 2


1) From what you've said, you should not need to replace the receiver/drier since it was never exposed to ambient air.

2) The oil tends to stay put for the most part unless the leak is large and sudden. You might want to add a little to compensate for what you see that leaked out, say 10-20% of the required amount.

3) Since you don't know the current charge you'll need to use temperature-compensated pressure readings to get it right. No real shortcuts here if you want it done right.

  • thanks Moab and jwh20. I'll wait until warmer weather to recharge. Not sure what I'll do regarding the oil. Short of having a shop evacuate the whole system (to ensure I remove all the oil), I'm not sure what other options exist. I'll probably just add a little oil when I add refrigerant. There was not much oil around the high valve so I imagine the system just lost a little.
    – redbeard1
    Feb 9, 2020 at 3:14

Purchase replacement refrigerant with oil already included. That would be easiest and get you to the approximate amount of oil that was lost.

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