I found a vacuum pump and drew the system down to about 23 inches last night, and left it there for 6 hours. This morning I took the pump off, and did a leak down, and I did not lose any vacuum at all. I checked it again at lunch time, and still had the same 23in~ of vacuum. I've read that 30in was the magic number to shoot for, but my pump does not seem up to teh task of pulling that much of a vacuum. My pump was a little low on oil, so I topped it off at lunch time, and hooked it back up and was able to get another 4in of vacuum out of the system, so I've drawn down to 27in of vacuum.

Can I get by with that, or should I go looking for a different pump to pull more air out?

quote Originally posted by topcat: I found a vacuum pump and drew the system down to about 23 inches last night, and left it there for 6 hours. This morning I took the pump off, and did a leak down, and I did not lose any vacuum at all. I checked it again at lunch time, and still had the same 23in~ of vacuum. I've read that 30in was the magic number to shoot for, but my pump does not seem up to teh task of pulling that much of a vacuum. My pump was a little low on oil, so I topped it off at lunch time, and hooked it back up and was able to get another 4in of vacuum out of the system, so I've drawn down to 27in of vacuum. Can I get by with that, or should I go looking for a different pump to pull more air out?

If it'll hold 27 inches for 6 hours I'd call it good.

Pumping a system down to a vacuum isn't really done to check for leaks; there's a more practical reason. At that low pressure, the boiling point of water and other contaminants is reduced to well below ambient temperature. This causes the unwanted stuff in the system to boil and the pump removes the "steam". This insures that your system is free of water - even one drop of water can prevent your AC from working. This is why we want to hold it at a vacuum for several hours; to give the contaminants a chance to boil off.

Of course, if it won't hold that vacuum for 6 hours it's got a leak that you'll want to fix before you pump it down again...

Thanks... yep, I know the reason for the vacuum is to get the moisture, not to find leaks... but my system is pretty tight. I replaced all the o-rings, the accumulator, orificace, hoses and flushed the system out.

I really feel like an idiot though. On the way home, I was thinking... "are the needles on my gauge set zeroed?" Well I walked into the garage, and looked at the needles, and I was way off zero with the pump off and the hoses open to atmosphere. I zeroed them, hooked the gauges up to check the vacuum, and had 29.5 inches on the system. I turned the pump back on for good measure to let it run for a few more hours before I have time to charge it up.

Thanks and if I had not already given you a +, you'd get another.

Cheers!

[This message has been edited by topcat (edited 06-27-2006).]