Leaking air compressor

[frankthezapper]

The old compressor that has lived under the bench for 30 years drops pressure over a not very long time, gets disturbing with the compressor kicking in. I have bubble tested all the connections and found nothing, could it be pressure leaking back past the valves? I imagine they are those flat sheet metal type valves, like reed valves we had on 2 stroke dirt bikes.

 

[Worsedog]

Generally there is a check valve where the line goes from the compressor to the tank. I'd look there, or around the pressure switch and pop-off valve. I've never seen one that relies on the compressor itself to contain the pressure.

 

[Leaflessshadetree]

30 years you may want to check it all over including the tank and drain valve. A dish soap solution and spray bottle might make it easier to pinpoint.
I'd start with the coupler and check every line connection.

 

[timgunn1962]

The Non-Return Valve where the pipe from the pump enters the receiver is the most usual culprit. It's usually a right-angle arrangement and there's usually a spring-loaded poppet in there made of a fairly hard rubber/plastic, seating on a knife-edge seat. The rubber/plastic poppet tents to harden up over time and develop cracks, which allow air to pass back towards the pump. Off the pump side, there is usually a small-bore line back to a valve on the bottom of the pressure switch. This valve opens when the pressure switch switches off and releases the pressure from the line between pump and NRV, so that the next time the compressor starts, it can get the first turn or so in before there is pressure for it to work against. The valve is open when the pressure switch is off and closed when the pressure switch is on: you'll usually hear a brief hiss from the valve every time the compressor stops. If you can hear a hiss from the valve when it's off, you need to change the rubber/plastic poppet (over here, Britain, they are known as pastilles, but I don't know what they'll be called in the US). Here, they used to be easy to buy but now it's harder to find them. A good pneumatics supplier can get them for a couple of quid, but most places want to sell a complete NRV (or a complete compressor).

 

[Skyman]

I leave the drain valve on mine cracked ever so slightly open. I can just barely hear a hiss as it bleeds down after use. I'd never remember to drain it after each use. Hell, I sometimes forget to shut it off.

 

[unslow1]

I've had two compressors develop leaks from the regulator knob.

 

[nadogail]

Soap Bubbles are your buddy when chasing leaks.

You might investigate the check valve between the compressor and the receiver; I like "Lift Checks".

 

[3rdgendslmech]

Once you checked all you're usual locations with soap water, try 1 more spot.....Check your air intake. Build up pressure and when the compressor kicks off put your thumb over the intake port ( just like you'd use your calibrated finger to measure compression on a lawnmower engine lol) and spray some soapy water around it and move your thumb around a little bit. If you have bubbles here your unloader valve is cracked and air is leaking back out of the intake.