Weekend find - 21 gal Air Compressor

[sam_i02]

Welp, picked this up over the weekend and excited about it as it fits perfectly in this spot I have in my 10x20 workshop. It is a Central Pneumatic 21 gal, 3 hp oil lubricated, 110VAC air compressor.
Owner bought it 6 years ago and it saw very little use. This summer he tried to use it and it does not pump up past 60psi (motors keeps turning, but pressure does not build any higher). He wanted $50 for it and it grabbed it.

I will do some diagnostics on it when I get home tonight and it may end up being a quick fix - or I may have paid $50 for a motor and a 21 gallon air tank.

Just some quick observations:
1 - when pumped up to 60psi and turned off, the pressure in the tank holds for 24 hours, so that puts a loose/leaky fitting out of the question.
2 - I am thinking it is either the pressure relief valve not seating correctly (which can be easily replaced) or the internal reed valve (which will be difficult to get a replacement for).

The model number is 47065 in case one of you owns a similar compressor and may have some input.

Cheers

 

[mayday0017]

I would recommend connecting something with it's own gauge as well to the tank. Wouldn't be shocked to find that the gauge shows 60psi but it's really at 100 or whatever...

 

[sam_i02]

Good point. I will certainly swap the gauge and check for accuracy. Although this is not the main problem, because it is the pressure switch (set at 110psi) that would be the turning the motor off when the tank reached 110psi, irrespective of what the gauge is reading.
Cheers

 

[sam_i02]

ok, could not wait till 6PM. Swung by the garage over lunch and did some quick diagnostics. Fired up the compressor and it came up to 60 psi in a couple minutes. Shut the power off and noticed a continuous hissing air leak sound (pretty significant). Putzed around with some soap water and found that is is coming from the unloader valve (picture attached).

Now I need help. In the pic, the copper line connects to the head and the plastic line connects to the pressure regulator.
Can someone please tell me what this valve does and should I simply replace it? Or is there a deeper problem that is causing this unloader valve to leak?

 

[Steevo]

That depends on where it is leaking from?
It looks like a check valve with a bleed-off hose that runs to the pressure switch.
What it does, is when the pressure switch clicks off, it pushes against a schraeder valve that is on the other end of that small tubing, releasing the head pressure between the pump and tank. This lets it start against zero pressure, so it can spin up.
It could leak from:
A fitting?
A gasket on the check valve end of it?
A crack in the valve body?
The check valve could be bad and not seating, letting pressure escape through the pressure switch, but that would only happen when the switch is off and the schraeder valve depressed.
You can find replacements at air tool stores, Grainger, etc.

 

[sam_i02]

Thanks Steevo.
Armed with this knowledge, I am ready to disassemble and start digging in to it.

 

[nine4gmc]

I have a cambell hausfeld compressor and it was leaking from the unloader valve. The guy at the parts store said I could try the valve but it was more than likely the check valve that goes into the tank. I took the unloader valve and tried it, same thing, still leaking so I went back and got the check valve that goes between the compressor out and tank in. That fixed me right up. Your compressor is different but my point is, it could be something else making it leak besides the part that is actually leaking.

 

[mayday0017]

My point is if it is shutting off at 110psi and the gauge is broke. Then it might only read 60psi but really be at 110psi, meaning there is nothing wrong with the compressor getting up to pressure, but more a problem with the unit showing proper pressure.... I have a theory of go after low hanging fruit first before trying to complicate things...

 

[sam_i02]

thanks mayday. makes sense. swapped gauges and it turned out to be right on.

took the check valve apart and found that the O-ring is too beat up to make a good seal. I believe that is the problem. Need to score a replacement and that may be the start of me using compressed air to serve me!

cheers

 

[bad_idea]

if you have trouble finding the o-ring, check with local hose repair and pipe fitting shops. should be less than a dollar. many shops have a minimum purchase cost though. call around.

 

[sam_i02]

yes finding an O-ring may take some time. I am going to use some of that nasty Toyota FIPG (Form in place gasket), give it a day to cure and see if it holds.

thanks for all the resposnes

 

[mayday0017]

Well what's the latest? Get this thing running!?

 

[sam_i02]

Yes it running...but I realized there may be additional issues with it.

Now after the the leaky O-ring is fixed, it starts up and beautifully pumps up to 80psi and then sits there and runs forever... I did some searching around and found that people with these compressors have found 2 additional sources of problem - one or both are likely affecting me too.

Other owners have taken the head off and found leaky head gaskets and dirty reed valves that do not seal properly. And some have found tears in the diaphragm at the pressure switch.

I will start checking these 2 areas for problems. But it will sit for a week as we are leaving town this Friday for a weekend camping trip.

Thanks for checking up on me. Cheap tools can be a PITA! Another option I have considered is finding an old Craftsman/Devilbiss compressor that has a rusted out tank, get that on the cheap and mount the motor and head to this tank and call it a day...

 

[Chattah]

I just got done fixing a compressor with the exact same problem this afternoon. I ended up having to fabricate three gaskets for the pump and that fixed it. When you take the head off my bet is you will see the gasket that seals between the intake and exhaust side has failed. A quick check to see if this is the case is to put your hand up to the intake and see if you feel air coming out instead of being sucked in once it reaches the point it just runs without building pressure.

I ended up using the rubberized roll of gasket material from Advance Auto Part it was 4.99 for the entire roll with tons left over for more jobs.