M929A1 Unloader Rebuilding - Is minor or major unloader kit required?

[Rockafellar Deuce]

So after a bunch of testing I have determined my unloader valve needs to rebuilt. My truck is really slow to pressure especially at idle. At driving RPMs it builds pressure just fine. Also, when I shutoff the truck I can hear air bleeding back into the air intake. Probably takes about 5 minutes or less for the primary tank to completely drain back into the air intake. So my question is, do I need a minor or major unloader rebuild kit for the Cummins #3018527 compressor (replaced by #3018534)?

I believe these are the correct kits.
Minor unloader kit... https://www.finditparts.com/products/363011/haldex-3559545k
Major unloader kit... https://www.finditparts.com/products/363015/haldex-3559551k

I'm hoping I only need the minor kit but any advice would be greatly appreciated.

 

[WillWagner]

Minor kit should do you fine UNLESS there has been coolant through it and the seats are pitted. Use STP on the o-rings. You should get the water grommets for the coolant pipes because they will leak if they are hard. There should be a check valve at the inlet to the wet tank. If the valve is failed, it will do the same thing. Unsure if there is a dryer on yours, but if so, there should be a check valve at the inlet of the dryer, and it is FUBAR, same thing will happen. Simp should chime in for clarification.

 

[simp5782]

So after a bunch of testing I have determined my unloader valve needs to rebuilt. My truck is really slow to pressure especially at idle. At driving RPMs it builds pressure just fine. Also, when I shutoff the truck I can hear air bleeding back into the air intake. Probably takes about 5 minutes or less for the primary tank to completely drain back into the air intake. So my question is, do I need a minor or major unloader rebuild kit for the Cummins #3018527 compressor (replaced by #3018534)?

I believe these are the correct kits.
Minor unloader kit... https://www.finditparts.com/products/363011/haldex-3559545k
Major unloader kit... https://www.finditparts.com/products/363015/haldex-3559551k

I'm hoping I only need the minor kit but any advice would be greatly appreciated.

You could have a stopped up compressor discharge tube as well.

Air bleeding thru the intake is going to be a vent from either the axles or the parking brake valve

 

[Rockafellar Deuce]

Minor kit should do you fine UNLESS there has been coolant through it and the seats are pitted. Use STP on the o-rings. You should get the water grommets for the coolant pipes because they will leak if they are hard. There should be a check valve at the inlet to the wet tank. If the valve is failed, it will do the same thing. Unsure if there is a dryer on yours, but if so, there should be a check valve at the inlet of the dryer, and it is FUBAR, same thing will happen. Simp should chime in for clarification.

You were correct, I pulled it part and the minor should be all I need. Valve and seat were gummed up a but it came off pretty easily. Actually wouldn't have even needed the minor kit if I had realized you could pull the valve up through the top cap to clean.

Also mine does have an air dryer, though it seems to working properly. At least when I connected shop air to the inlet side of the dryer. Air moved through it fine and filled all the tanks quickly. Though I didn't remove the shop air, I just turned it off and the tanks all maintained 120 psi without leaking down. I don't remember seeing a check valve at the dryer inlet. Though it could be located on the frame rail. If the unloader valve cleaning doesn't work I will look for the check valve. Thanks for your help.

 

[Rockafellar Deuce]

You could have a stopped up compressor discharge tube as well.

Air bleeding thru the intake is going to be a vent from either the axles or the parking brake valve

I hooked up shop air at the compressor discharge line and it fills the tanks quickly, so I don't believe it it plugged. I will look into the parking brake valve and axles if the cleaning the unloader valve does not solve the problem.

 

[WillWagner]

If you charged the system and nothing came back out the compressor discharge at the compressor, the check valve is fine

 

[Rockafellar Deuce]

You could have a stopped up compressor discharge tube as well.

Air bleeding thru the intake is going to be a vent from either the axles or the parking brake valve

Well I received the minor unloader kit yesterday and installed it. I'm still getting air bleeding back into the intake. The unloader seems to working as it should, nothing bleeding though there. I must have a leak coming from the axles or parking brake valve like you said. I will have to check them next.