Champion R15B Centrifugal Unloader Valve Won’t Close

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by Greg White
(Houston, TX)

I’ve posted this on a few forums and so far I haven’t been able to get any help at all. I’m hoping I’ve finally come to the right spot!

Champion R15b Centrifugal Unloader Valve
Champion R15b Centrifugal Unloader Valve

I have an air compressor that I bought used from the Army at a surplus sale. I only paid $200 for the whole unit and it was OD green and had some surface rust so I figured I’d take it apart, clean it up, paint everything, replace the rusty fasteners and fittings, then put it back together.

Champion R15b Centrifugal Unloader
Champion R15b Centrifugal Unloader

I had to buy a single phase motor for it and I built a little sound enclosure shed for it behind my garage as well. I did all that and everything is great with the exception of my centrifugal unloader and what may be an oil leak.

Champion R15b Centrifugal Unloader Valve Issue
Champion R15b Centrifugal Unloader Valve Issue

The centrifugal unloader on the front of my pump has a breather that the Army TM (TM = Training Manual, or the tech manual. I’ve put the link below it in case anyone needs it since it covers Champion R15 pumps well) calls an unloader body. Looking at the old rusty one I think it’s just a 3/8″ NPT breather so I got a new one seeing how the old one was rusty and the felt screen was worn out. When I turn on the air compressor it starts just fine and doesn’t draw too many amps. My understanding is that once the pump reaches a certain RPM that the weights move outward therefore taking pressure off the pin and allowing the little valve ball to seat and block off air from escaping through the breather I described earlier. The problem with my compressor is that when I start it and let it run the unloader valve never closes and therefore air is constantly shooting out the breather. As a result the tank never fills completely with air and the whole unit

just runs and runs. I put a 3/8″ NPT plug in place of the breather, the tank pressure shot up to 175 psi in no time, and then the motor turned off like it should. I have included a picture of the unloader I bought that is installed and some overall photos of the whole air compressor unit as well. I’m thinking that for some reason the weights are not swinging out and therefore the valve can’t close. I removed the little plug that holds the spring and ball, made sure they were clean, and then made sure the pin that pushes them was moving freely. Everything seems to be working fine, I just can’t figure out why this valve won’t close.

The other issue I’m having is with some oil leaks. I originally used Catoil (Link to tech specs: as I was told it was a top notch air compressor oil. Based on those numbers it looks like it may not be heavy enough so I drained that out and replaced it with some regular 30W non-detergent oil. When I was using the Catoil the compressor leaked and slung oil everywhere like crazy. With the 30W ND I’m noticing some but the pump hasn’t ran much so I’m hoping that it’s just getting the rest of the Catoil out of it’s system. I also had a thought that possibly the Catoil was so lite that it got into the piston and somehow affected the RPM of the pump. I’ve noticed the oil coming out of the manifolds but when I check them for leaks with soapy water I don’t get any bubbles. Could oil in the pistons effect RPMs in a way that would cause the centrifugal unloader to not work properly?

Sorry for the long post, I just want to make sure I include as much detail as possible.


by: Doug in

So, you replaced the motor.

Is the new motor turning the pump at a speed in range?

My first thought reading this is that the pump is simply running too slow to force the weights out…

Let us know, would you?


Reply to Doug in
by: Greg White

Thanks for the reply Doug.

The motor that came with the air compressor was a Marathon 5 HP, 1740 RPM, 3-phase electrical motor.

I replaced it with a Leeson 5 HP, 1740 RPM, 1-phase motor.

I’m no expert on electrical motors but I would imagine that these would spin the compressor at the same RPM.

I am using the same pulleys that came with the air compressor when I got it.

I also understand that the original motor was 3-phase and this new motor is 1-phase but I think that means that the 1-phase motor will just consume more electricity than a 3-phase motor.


by: Doug in

Greg, that’s pretty much it. I’m gonna assume it’s rotating CCW, although that wouldn’t affect the unloader operation.

So, I don’t have hands-on with this, and all I can see is maybe the springs are different and reversed?

Failing that, maybe try leaving the outer spring out to see if it closes that way?

(You do know it’s not dirty, yes?)

Keep us posted, OK?


2nd Reply to Doug in
by: Greg White

When you say the springs are you talking About the springs on the centrifugal unloader? The two springs are very different in size so it would be impossible to mix the two up. Or do you mean putting them in, actually backwards?

The pump spins clockwise when looked at from the front, as it is in the photo. If you were to look at it from the rear, or the flywheel side, it rotates counter clockwise.


by: Doug in

Yes, the rotation is correct.

As to the springs, I did mean transposed as opposed to backward.

So, back to the valve –

Can you move all the bits that are supposed to move manually to see if the ball moves properly, without binding?

Also, am I correct that this is essentially the old valve, except for the exhaust port?


3rd Reply to Doug in
by: Greg White

Yes, I removed the bushing that the spring and ball retract in to and they move freely. Also, everything in the centrifugal unloader is all original with the exception of the breather and the gasket.


by: Doug in

Well, I’m out of ideas.

I would just put a valve on the breather barely cracked open (or drill a small hole in a plug) for now, and wait for a brainstorm or maybe someone else will read this and have a solution.

If/when you figure it out, let us know. Sorry can’t be more help, good luck.


Champion R15 pump unloader…leaking air
by: Dennis Frank

Your Problem i believe is this …air comes from the unloader all the time…and oil is slung everywhere.

These are signs that your CHECK Valve is worn out. Therefore air can escape tank through the crankcase and also that amount of pressure in the crankcase will BLOW the oil out.

Try Replacing the check valve…i think your problem will disappear…also unplug the centrifugal unloader outlet so it can the unload properly. Hope this helps ..sincerely..Dennis Frank..[Frank Air Compressors]


Thanks Frank!

I tried to get a link to Frank air compressors for posting here, and found two. Don’t know which is for the poster, but hope one of them is:

Screw Air Compressor Manufacturers


Reply about check valve
by: Greg White

The check valve is brand new actually. I’m thinking it may be that the new rings need to get broken in. I’m going to run it for a few hours and see if that helps.

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The unloader pin connecting to the centrifugal assy is sticking in the low speed position, this is usually caused by lack of use or debris, you can remove spring and ball then oil the pin and function a dozen times in/out with a screwdriver or similar until it’s free. Clean and oil then you are good ??

i am having very close to the same issue with the oil in the valves. after removing the air filter i can see ALOT of oil is being blown into the intake from the unloader. the HP tube was already removed when i was left this compressor(dosent make up for the almost year worth of rent the guy ows me but thats a diffrent story), no unloader at all on it.. im turning the pump with a 3HP 3 phase motor with a 3phase to single phase VFD driving it($100 for 3Hp about $150 for 5HP, much cheaper than replacing… Read more »

disconnecting the pump output from the tank stopped the oil leak so it is the bad check valve. thank you for sharing your story, it allowed me to fix mine.

” i also know for a fact the check valve IS bad BC i can pull the pop-off and will keep releasing air. could it still be the problem even without the HP tube?”
Possibly. But I’m having trouble visualizing what is connected or not, and where…could you send some pix so we can see?

Hey Greg White –
Anything new with your R15? Our old thread is on this page.