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Need help pulling a vacuum...

So I am pulling a vac on a small 1.5 ton system with a new JB gauge and JB 7cfm vac pump. I can get down to 50 microns but it jumps back to 1200 then slowly drops to 900 and holds. If I turn it back on, it is immediately at 60 microns. I pressure tested with nitrogen to 200 psi and it held for 30 min solid.

Any thoughts????

Thanks a ton! (or a ton and a half!)
Just a guy running some pipes.


  • Timco
    Timco Member Posts: 3,025
    So I re-pressurize

    But now to 300psi. Then I leak test all joints. No leaks found. This is really frustrating to say the least....
    Just a guy running some pipes.
  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,549
    just a guess

    Maybe it has a bit of moisture that's hiding in there. That 7 CFM may be pulling so fast, since it's such a small system, that it looks like the system is down to 50, but it really isn't. Let it run over night?
  • Timco
    Timco Member Posts: 3,025
    The All Nighter...

    Always liked Glenn Fry.

    I didn't know running all night was a possible necessity. Thanks for the tip. I'll do that. I'm at 50 microns now.
    Just a guy running some pipes.
  • Timco
    Timco Member Posts: 3,025
    Having the worst time.

    So I got the last small system to finally pull down and stay down. Now I get a call about no AC and find the system flat. No pressure at all. Chage 300 psi nitro and zero leaks. Now after 5 hours of pulling down it hits 140 microns and jumps to 2400 when I isolate the pump, then it drops back to 1200 and holds.

    Moisture AGAIN? Is it common with a 7 cfm pump to need to pull down all day? This is a 2.5 ton system, 30' lines.
    Just a guy running some pipes.
  • don_9
    don_9 Member Posts: 395
    edited May 2012

    If it has one you might be getting beatup bc of it.Replace the drier every time you open them up.

    How your vacuum system setup?.I have found that my gauge connected to one service valve and my copper line from my vacuum pump to the other service valve with a shutoff valve helps with keeping the problems down you are experiencing

    Got away from using refrigerant hoses along time ago when pulling a system down.Usually if there is a leak she going to have a very hard time pulling down at all.
    RDSTEAM Member Posts: 134

    is this a new install or a replacement?? typically when you pull down and isolate the pump valve your microns will go up. an allowable limit is 1000 microns. if it stays below that your good. as far as the flat system goes, i'm assuming you pulled down and then filled with refrigerant. did you check pressure will running it to make sure the unit even had charge in it (maybe you got a bad condenser). if you have a leak detector put a trace charge in it..... good luck
  • Techman
    Techman Member Posts: 2,144

    If the system was flat when you went back ,then there is leak somewhere . And then pulling a good vacuum is kinda hard.
  • Bart Vaio
    Bart Vaio Member Posts: 56
    I can pull vacuum on leaking systems all day long

    I have done it a few times, pulled down passed 500 charged system and returned later to find no gas.  Only one kind of leak will pass a pressure and Vacuum test.  The leak is hiding under your hoses low loss fittings when charging or vacuum, check and replace your schraders.  You can put a 50 cfm pump on and it wont matter, your cfm restriction is your hoses
  • Timco
    Timco Member Posts: 3,025
    Ding ding, we have a winner....

    So I went back 3-4 weeks later on a no AC call, same unit. I swear to you all, I had a great vac pulled, no leaks with the nitrogen, and ran great when charged.

    NO GAS.

    Pulled valve core, liquid side, no seal. One $.02 stem caused all that.

    Some things you just have to figure out on your own. Thanks for all the great advise.

    Just a guy running some pipes.
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