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Discussion Starter · #1 · (Edited)
I have a 1940 Chevy Special Deluxe with a Chevy 350 engine. I have been working on this car for THREE years and finally its drivable. One of the last issues is the air conditioning. This is a new build, Vintage Air AC system with lots of custom parts.

The issue I have right now is I cannot pull a vacuum below 20 inHg. Its been running now for over an hour and I'm just at 17 inHg. I have been trying to troubleshoot this issue for weeks. I put the system under pressure with about 100PSI and did soapy water at every connection and no leaks. I left it at 100psi for over and hour and it just dropped very little which makes me think I am leak free.

So why on earth cant I pull a decent vacuum? What am I missing here?
 

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47 Plymouth Special Deluxe
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178 Posts
The issue I have right now is I cannot pull a vacuum below 20 inHg. Its been running now for over an hour and I'm just at 17 inHg. I have been trying to troubleshoot this issue for weeks. I put the system under pressure with about 100PSI and did soapy water at every connection and no leaks. I left it at 100psi for over and hour and it just dropped very little which makes me think I am leak free.

So why on earth cant I pull a decent vacuum? What am I missing here?
Hello Copperline,

I know how frustrating AC work can be, hang in there.
When you get to 17inHg, and you turn off the vacuum pump, does the system hold there? If not, how quickly does it leak down? Or are you just unable to get to 30 inHg? Have you tried a different vacuum pump?

Let us know, thanks.
 

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Discussion Starter · #6 ·
When I posted this about two hours ago i was at 17 and shut off the valves. When I came back to the shop just now I'm now at 15.2. I'm not sure if that is an actual leak or just the residual leaking of having all the hoses and fittings on the manifold set.
 

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1949 Ford Coupe RESURRECTION
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Your vacuum pump may be screwed, or low on oil or needs an oil change or is worn out or made in China.... put your hose connections on the "holder" (blocked) and see if you can get to 30" NOT hooked up to your system. And you need a leak detector while temporarily pressurized with refrigerant. Bubble test isn't good enough
 

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1949 Ford Coupe RESURRECTION
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1,143 Posts
OR your gauge is screwed... And by the way when you do get it to pull down where it should be break the vacuum with refrigerant on the high side port and then blow off the pressure on the low side and evacuate again and do that twice. Do you see what that does to get the moisture and the air out of the system?

Also pressurize it that last time note the pressure, close the high side valve to the gauge...do not close the low side valve... and make sure all connections are good and tight and leave it overnight. If the pressure has not changed one bit the next day then you're good to go. But it's always best to go over the whole system with a real leak detector. You can get a Chinese one off of Amazon pretty cheap.
 

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Discussion Starter · #11 ·
I changed the oil in the pump and still wont go below 17". I took the pump apart and cleaned it (not too bad) and still 17". I don't think my gauges are bad as this is a digital Fieldpiece. I think the pump is just tired. Ive had it for easily 5-6 years and have done a bunch of cars and house systems. I went ahead and ordered a new pump so we will see if that really does it. I do have a freon detector so once i do get this charged up then ill be sure to use that.
 

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I changed the oil in the pump and still wont go below 17". I took the pump apart and cleaned it (not too bad) and still 17". I don't think my gauges are bad as this is a digital Fieldpiece. I think the pump is just tired. Ive had it for easily 5-6 years and have done a bunch of cars and house systems. I went ahead and ordered a new pump so we will see if that really does it. I do have a freon detector so once i do get this charged up then ill be sure to use that.
If you pressureize it don't use shop air too much moisture. Use nitrogen.
 

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I just got done redoing the a/C in my '39. I had issues too. Here are a few things I found. First my gauge set was old and as it turned out the rubber lines were cracked and leaking. New set of gauges. Several times I found a leak on the gauge hose connections. Pay attention to details like this! Found one crimp that was leaking. Normally you look for leaks at mechanical joints but this one was back on the sleeve. If you air load the system and it looses one pound over a two hour period you have a leak somewhere. Same goes for vacuum. Really hard to find the small leaks and sometimes it may be necessary to add dye.
 

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I have done my own A/C work on my 2010 GMC by replacing the compressor, condenser (in front of the radiator), accumulator/dryer (it actually has vermiculite in it) and some hoses. I purchased a Harbor n Freight vacuum pump and it work very well. I had no issue reaching and holding the max vacuum on the gauge (I let it sit for an hour and no change). If you can not reach and hold a hard vacuum, there is a leak in the system. If could be any part I just mentioned, but God help ya if its the evaporator buried in the dash.
 
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