M923 Air Compressor Rebuild thread advise

[Elijah95]

Howdy folks, hopefully everyone’s year is off to a good start.

I’ve got an issue I’m struggling with on my M923A2, I’ve rebuilt my air compressor 3x.... yes, it’s getting old. Nothing special, just an SS296

Back story: air compressor Head was gummed up with crud, and it was sticky. Left me stranded in Baton Rouge during initial recovery in 2017. Used some compressed air, PB blaster, and got it freed up. No problems for a while.
20,3xx miles

1st Rebuild: was going through the power steering pump, fixing fuel leaks etc and pulled the head off the compressor, and separated the unloaded head from main head, rebuilt with all new gaskets, O-rings, and off to Texas for Hurricane Harvey I go.
24,xxx miles


2nd Rebuild: truck survived Harvey, went to GA Rally (noticed it was building air very slow) then went to Montgomery Truck Meet and it suddenly quit pumping when I pulled in the gate. Huge thanks to Wes towing me home!
Disassemble, find headgasket-unloaded gasket blown, check head on flat surface... that’s what it was.
Flat file it until it’s all flat, clean everything up, new hardware new springs new everything goes in to this Rebuild. All is perfect
27,xxx miles


3rd Rebuildcurrent as of OP)
Truck suddenly quits building air W T F
Do my typical “disconnect governor line from compressor head, squirt lube in hole, reconnect line and normally it works just fine”
It didn’t.
Scavenge some compressed air, blow down through the governor signal port, works fine to limp home. Order all new gaskets see # PAI 131430. Upon tear down, I find all gaskets are in great shape, but the 1 way O ring seen here was cracked and the sliding ?piston? that rides on it was all gummed up from the penetrant oils sprayed down the governor signal line and cooking inside head with no escape. Lesson learned
29,xxx miles

Okay, build it by the book again, at this point it has New Gaskets, O-Rings, Springs, etc spend 2 hours with sand paper strapped to machine flat surface slowly working the head until it’s flat, and ever so slightly roughed up to give grip on gasket. Then do the same thing to unloader plate that goes on top of the head. Next, lube up the O-Rings with a small dab of food Grade silicone grease, WOW it works better than ever! But I don’t trust it, I plug the ports off and do a compression test on the compressor, it’s within spec, cross hatching still intact, no carbon in chamber, should last 100,000 miles right? WRONG. Today a couple thousand miles later, I can hear it slowly engaging and not building air as fast. I know it’s going to go down again.


Tips, tricks, suggestions?
-Verify Head flatness
-Order new parts again
-clean all metal parts and polish slightly
-torque head bolts to spec with light lube on threads

What could cause a failure after going by the book, governor? Drier? A curse? I’m a competent mechanic, I can rebuild an engine, a transfer case, an automatic transmission, but this thing has me at wits end. Buying a new compressor is Not an option unless someone has a reman they can sell for cheap.

 

[sandcobra164]

Eli, the compressor can work great and the brake treadle valve can let you down. It happened to me recently. I was taking my truck over to Welder1's shop so he could outfit my truck with a nice flag bracket. I had to do a panic stop, no worries, brakes did fine. Afterwards my air pressure was about 70 which seemed very odd. I decided to press on and not use my brakes unless absolutely needed to save air for a stop. Got to his shop with about 50 psi and the rear drums were hot! The next day I drove through the parking brakes as the truck was not building air and we hooked it to shop air through the rear emergency glad hand. We couldn't find the leak but I furiously stabbed the brake pedal out of frustration as it was venting to the intake stack. We heard a pop, disconnected the air hose and drained the tanks. Started the truck and it built air pressure again. We did this 2 days before the Ga Rally last year and it's been fine ever since.

 

[Elijah95]

Eli, the compressor can work great and the brake treadle valve can let you down. It happened to me recently. I was taking my truck over to Welder1's shop so he could outfit my truck with a nice flag bracket. I had to do a panic stop, no worries, brakes did fine. Afterwards my air pressure was about 70 which seemed very odd. I decided to press on and not use my brakes unless absolutely needed to save air for a stop. Got to his shop with about 50 psi and the rear drums were hot! The next day I drove through the parking brakes as the truck was not building air and we hooked it to shop air through the rear emergency glad hand. We couldn't find the leak but I furiously stabbed the brake pedal out of frustration as it was venting to the intake stack. We heard a pop, disconnected the air hose and drained the tanks. Started the truck and it built air pressure again. We did this 2 days before the Ga Rally last year and it's been fine ever since.

Thanks for the info, but I’m not having air leak issues, I’ve been having issues with failure to build air/slow to build air/delayed function of unloader kicking in fully to allow building of air

 

[simp5782]

Replace it with a remanufactured unit. Quit wasting your time rebuilding it. Time is money. Repeat repairs is not proactive

 

[Elijah95]

Replace it with a remanufactured unit. Quit wasting yoirntome rebuilding it

Do you know of any cheapies floating around? I cringe at a $200 core charge

 

[simp5782]

I'll ask around rookie. I can prob find one. It's a 296C. Not a standard 296. It is the only 11 tooth thru for pump.

About 285$ for a reman or 550 for new. Or you can go for an upgraded 18cfm unit here. https://www.finditparts.com/products/6610682/bendix-9111539202x


Oh and quit spraying crap in your unloader. You were told not to. It does nothing but gunk it up.

Do I need to come tow you to Ron's memorial service so you can attend? Ron would get a kick out of it since you said you would tow anyone who broke down on in.


That other unit is cheaper with a cheaper core. I can check around Monday. Or buy a junk core off Ebay and throw it in the return box

 

[simp5782]

You also need to go to a larger discharge tube and fix all of your air leaks. It basically is just carbon backing up from it having to work to air the truck up and I am sure you are the typical youngin and out hoping from here to there. Stop and go. Air tanks leaking all the air down every 5 mins it sits still. Basically. Alot of air leaking equals more wear and tear. Yea. Few miles but the conditions you are giving it simulate lots of more mileage. When I am driving at highway speeds my air dryer may purge off every 45 to 50 minutes. I have timed it several times. Mine is so slow leak off air for the compressor to come on that i had a small bit of water in an elbow going to a wet tank freeze up while the truck was idling cause no air passed thru that elbow until after it froze. Maintenance. Fix your air leaks. Take 1 day a week and go thru every valve and fitting in a certain section. Take the hoses loose. The port fittings out. Teflon tape them and reinstall. Cut the farrels off the line. Or buy new farrels and line which are cheap and re do your air lines.

Also the cotton jacket air lines from the treadle valve that go under the cab tend to leak thru the cotton jacket and you cant hear or see it just bubbles when you spray it

 

[doghead]

Are you 100% sure you have coolant circulation through your compressor?

It looks like it has been overheated.

 

[Elijah95]

You also need to go to a larger discharge tube and fix all of your air leaks. It basically is just carbon backing up from it having to work to air the truck up and I am sure you are the typical youngin and out hoping from here to there. Stop and go. Air tanks leaking all the air down every 5 mins it sits still. Basically. Alot of air leaking equals more wear and tear. Yea. Few miles but the conditions you are giving it simulate lots of more mileage. When I am driving at highway speeds my air dryer may purge off every 45 to 50 minutes. I have timed it several times. Mine is so slow leak off air for the compressor to come on that i had a small bit of water in an elbow going to a wet tank freeze up while the truck was idling cause no air passed thru that elbow until after it froze. Maintenance. Fix your air leaks. Take 1 day a week and go thru every valve and fitting in a certain section. Take the hoses loose. The port fittings out. Teflon tape them and reinstall. Cut the farrels off the line. Or buy new farrels and line which are cheap and re do your air lines.

Also the cotton jacket air lines from the treadle valve that go under the cab tend to leak thru the cotton jacket and you cant hear or see it just bubbles when you spray it

Yeah come pull me for good times sake.

I quit squirting stuff down the line a rebuild back. Sounds like a plan to me. My truck will hold air for about 12 hours, but moving down the road I have something leaking that isn’t my brakes that causes my compressor to cycle about every 5-15 minutes

 

[Elijah95]

Are you 100% sure you have coolant circulation through your compressor?

It looks like it has been overheated.

Sure am, went through my cooling system cleaning when the headgasket blew last year, and I now run a coolant filtration system. Compressor coolant lines are clear and flowing