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  1. #1
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    Nov 2004
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    Default Air compressor holding air

    I've got a GMC 2.5 hp 40 litre tank air compressor. I've read where someone can fill the tank and the air remains in it for days without having to turn on the noisy compressor and fill it up again. It's been suggested that my compressor may have a leak as it doesn't hold air for more than 10 mins or so after I turn it off. Now before I conduct an exhaustive search on this, are these air compressors that are at the cheaper end suppose to hold air for hours afterwards? I have teflon around the fittings maybe that's not enough?

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  3. #2
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    Aug 2005
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    Default

    Check your fittings. Tape or not there will be leaks.

    Fill a a spray bottle with water and some detergent and spray the fittings,connections etc. Bubbles will tell you where the leaks are.

    Use "quality" fittings eg Nitto rather than the "Nitto type" as the El-cheapo ones are false economy.

  4. #3
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    I'd start at the compressor as this is where the vibe is the greatest and I used to get leaks.
    Disconnect any hoses and charge it up and test as per BobS's suggestion.
    Don't forget to check all the junctions that connect to the tank and the drain valve as well.

    Once that is sorted check the hoses. I find the worse offenders are those thin coiled plastic hoses with the crimped end fittings

    Standard white teflon tape doesn't like being near oil so I'd replace that with the yellow gap pipe stuff or a liquid thread sealant which are a bit more oil resistant.

  5. #4
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    Nov 2004
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    Thanks to the Bobs, I've had this compressor for years and never even considered that I wasn't using it to its full capability. I will start checking it, how do I then know that it's working as it should? Can I start it up at 6pm and then turn off and expect it to still have enough air to run it at 9pm?

  6. #5
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    Quote Originally Posted by Tiger View Post
    Thanks to the Bobs, I've had this compressor for years and never even considered that I wasn't using it to its full capability. I will start checking it, how do I then know that it's working as it should? Can I start it up at 6pm and then turn off and expect it to still have enough air to run it at 9pm?
    If it is perfectly sealed it should never loose air.
    This can become a bit of a game/competion.
    A mate of mine spent a lot of time going over all his junctions with a fine toothed comb and claims his can hold full pressure for at least one week.
    I stopped checking after mine lost no pressure for 6 hours but I notice it now seems to have a small leak.

  7. #6
    Join Date
    Aug 2013
    Location
    Jerrabomberra, NSW
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    Default

    Hi,

    I have a relatively cheap compressor from Super Cheap and it holds air for many weeks. However, if I leave the tap on to allow air to the flexible line, it goes away in days. If you don't have a tap to isolate the outlet; try comparing the holding time with the air line disconnected vs connected. Then, as suggested, use a soapy mixture to find the leak, just as you would with a leaking tyre.

    Bruce

  8. #7
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    Aug 2008
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    As mentioned above, it's basically a game of 'how long do you want to spend testing every single fitting and connection', but it can be done.

    We've got a portable compressor at work that gets dragged out when we do large on-site jobs - MAYBE once or twice a year - and it doesn't leak. At all. It's been sitting in the corner for months now with about 7 bar in it.


    I just wish our proper air system in the factory was so well sealed

  9. #8
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    Apr 2011
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    Dandenong, Vic
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    My cheapie one also started kicking on and off every few minutes.
    I found that the 4 bolts that hold the head down were getting lose.
    I tightened them down and its all good again (well for 3 months anyway).

    Then I tighten them down again.
    Should locktight them down.

  10. #9
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    Nov 2004
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    I had become distracted by a few other things but got around to testing with soapy water and found that indeed there was a leak on the connection from the compressor to the air hose. I tried numerous times to reinstall the teflon tape without success, the soapy water test continued to show a leak. Bob mentioned a pipe sealant, i assume i can use this instead of teflon tape? It would be far easier to apply, any brand do?

  11. #10
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    Quote Originally Posted by Tiger View Post
    I had become distracted by a few other things but got around to testing with soapy water and found that indeed there was a leak on the connection from the compressor to the air hose. I tried numerous times to reinstall the teflon tape without success, the soapy water test continued to show a leak. Bob mentioned a pipe sealant, i assume i can use this instead of teflon tape? It would be far easier to apply, any brand do?
    It sounds like you have a damaged or stretched thread. I would start with a close inspection of the thread and maybe changing the hose flitting. See if you can find a quality fitting.

  12. #11
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    It's probably more about my teflon tape application Bob rather than a bad thread but rummaging through my stuff i found some gas pipe thread sealant, had it for a while might give that a go.

  13. #12
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    Nov 2004
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    After a few attempts I have now got the connection from the compressor to the hose right, i can't detect any leak now, however the connection from the air hose to the blow gun still leaks. I've tried a pipe sealant and numerous teflon applications but the bubble test still shows a leak. The thread on the connector to the air gun is only about 6 mm long, the teflon tape is approx 12 mm wide, do you need to cut the teflon tape in half, is there some other way to apply it?

  14. #13
    Join Date
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    You can start by folding it over a bit and stretching it as you apply it. You only need two or three laps, max. The thread of the male Nitto is not straight, but ever so slightly trapezoidal. Wind that bastard in hard. BIG spanner!

    I was originally going to reply that the other place to loose pressure is the simple one way valve thats inside the banjo going into the tank itself. If you take it apart you will find a plug that sets like a tap washer and behind this is a meaty spring. If this doesn't seat/seal properly, all that air is going to leak past and into the piston where it will leak down/past the rings.

    Here is a typical one pictured. Everyones a bit different, but the principal is the same.

    central-pneumatic-compressor-40440-good-workhorse-with-cheap-parts-check-valve-seems-to-be-one-o.jpg

  15. #14
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    Quote Originally Posted by Tiger View Post
    After a few attempts I have now got the connection from the compressor to the hose right, i can't detect any leak now, however the connection from the air hose to the blow gun still leaks.
    What sort of blow gun is it? Most cheap guns are made from recycled 50 year old dinky toys and are notorious for stretching or splitting at the thread.

  16. #15
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    It's a cheapish one but I haven't noticed any splits along the thread, will take a look tonight.

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