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Correct vacuum pump usage and leak detection

geezee on Tue July 29, 2008 10:08 AM User is offline

Year: 1992
Make: Honda
Model: Civic
Engine Size: 1.6 EX
Refrigerant Type: r134a
Country of Origin: United States

Hey all,
a couple questions on vacuum pump use and leak checking (w/o refrigerant)
I'm in the process of evac'ing a newly rebuilt AC system in the Civic, I'm attempting to 'leak check' (via vaccum) - prior to charging with r134a.
IS it correct to 'leak check' with vacuum ?
If so how, since the hi/lo valves are designed to hold a fairly high amt of internal pressure , not vacuum which is the exact opposite ( granted at 20-some inches of vac, theres not that much pressure differential there) is leak checking with vacuum valid ? Will the hi/lo valves leak under vac ? Maybe ?

As a very occasional user of pump and gage set, I did the 'loaner' thing from Autozone. The gage set is mastercool, I forget the vac pump mfr. The max the pump will pull is a tad over 20" - yet everything I see says pull a vac of 29" or so on the system for 30+ min.... What gives, is this pump a p.o.s. or am I doing something wrong ?! The fact its a auto parts store loaner of course has me leery of the pump... Secondly, I've never used one, so I'm leery of me, too. I did do the little 'ballast knob' 1/4 turn thing as per the pump startup instructions, however I see nothing else I'm doing wrong, but I don't get what I understand to be the full vacuum of 29" I hear is possible....

Also, if I pull this 21" vac on my AC system, then disconnect gage set from the car, cap the two valves on the ac lines and leave the system 'sealed' (presumably)overnight, the vacuum is almost completely gone in the car ac lines by morning. (my crude check is depressing the low side valve stem and noting it hardly sucks any air in, whereas immediately after disconnecting the vac pump, if I do same it has a noticable 'hiss' as it inhales).

My system will not hold a vacuum overnight.

hope this isn't too ignorant a question(s) !

mk378 on Tue July 29, 2008 10:31 AM User is offline

Leave the gauges and hoses connected to the car during the test, as then you won't have to rely on the shrader valves to hold vacuum. First test your manifold with it disconnected to make sure it holds vacuum.

The pump should be working a lot better. Is it properly filled with oil?

webbch on Tue July 29, 2008 10:42 AM User is offlineView users profile

You can verify that your vacuum pump is pulling a vacuum and holding. If it doesn't hold then that indicates a leak. If it DOES hold, it's very LIKELY that there's no leak, but not a guarantee. I'm no pro, but on my first major repair, I pulled a vacuum down to 500 microns and it climbed up to 750 microns in 30 minutes. My initial conclusion is that I probably didn't have any leaks. However, wanting to try out my new nitrogen tank, I then pressurized the system with nitrogen and it was leaking like a sieve (hissing) from an o-ring I had damaged when re-attaching the lines to the evaporator. A quick lesson illustrating to me that just because the system holds a vacuum doesn't guarantee it is leak-free.

Yes, the system should definitely hold a vacuum overnight. I pulled a vacuum on my friend's car one weekend, but didn't get around to charging it until the next weekend. I knew I would have to pull a vacuum again on it because of this, but I noticed when I hooked up my gauges (a week later) to it, There was still a substantial vacuum (like about 20-21 in. Hg. still). Obviously insufficient for charging into, but just to give you an idea of what can be expected. This was a 93 mustang, by the way, with the dreaded Ford spring lock connectors known for leaking terribly ;-)

The loaner pump is definitely shot IMO. It should EASILY pull down WAY better than that.

With regard to leak detection, the vacuum test will only tell you if a leak exists. Finding it can be done several ways, in order of increasing cost of tools)
1) UV dye in your refrigerant when you charge it. Use the glasses and low light area to check for leaks (typically good for larger leaks)
2) Charge the system and go searching with an electronic leak detector
3) Charge with a small amount of refrigerant, then pressurize the system to about 150 psi with nitrogen, and go searching with the leak detector (preferred method for those hard-to-find leaks).


Edited: Tue July 29, 2008 at 10:57 AM by webbch

geezee on Tue July 29, 2008 11:00 AM User is offline

The manifold will bleed off vacuum pretty rapidly when I turn the pump off. Should the quick disconnect valves on the manifold hoses seal (close) when NOT connected to a shrader valve ? Cuz they don't fully seal when disconnected. If I turn the pump off, the vac bleeds off back to ambient in about 5 seconds. If I turn the pump 'on' with it connected to the manifold but completely disconnected from the car (both valves 'open' on the manifold) the vac jumps quickly to 21 (and no higher), and when I turn it off it bleeds back to 0 in about 5 seconds. (again, this with the manifold connected only to the pump and not to the shrader valves.) So maybe the pump can only get to 21" because the manifold leaks.... I dunno, I'll have to fiddle with it this evening.

And I do still know my ac system itself won't hold a vac for 24 hrs. So maybe some type of pressure test (listen for the 'hiss' ) would work for finding a gross leak ?


mk378 on Tue July 29, 2008 12:38 PM User is offline

Even a good vacuum pump won't hold vacuum while it is switched off. When you're done pumping, close the valves on the manifold to isolate the pump, then turn it off. Some pumps have an isolation valve on the inlet fitting where the hose attaches.

iceman2555 on Tue July 29, 2008 2:30 PM User is offlineView users profile

A system that will not 'pull' a good vacuum and a system that 'bleeds' off vacuum at a rapid rate once the pump is shut down.....seems like there is a serious leak in the system.
Suggestion would be to introduce one can of refrigerant into the system...and attempt to locate the leak....a leak this large should not be hard to locate.
Good luck

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