Air Dryer

I know I will get flamed for posting this but if your air dryer lets you down on your M939 series truck, you are done!!! I drove my M923 a few days ago without issue, tonight, it will not build air. I started it up, heard the compressor doing its job and boom, expello valve blew off like always... So I thought... It will no longer close. I tried to push up on it, no luck. I shut the truck off and drained the air tanks, no luck. I disconnected the primary air tank line and pushed up of the valve, no luck. I contacted a member on here who has an updated air dryer for less than I would spend on a rebuild kit and will install one of those next week. I really wish there was a fanstastic work around to the issue but M939 series trucks need air to get the brakes to work. I have plenty of caging bolts.

I had an issue with mine today as well. I luckily had a 5/16 square and I drained it and it acted fine. Why could you not rig a line off the one on the drivers frame rail and hook it into a tank? would it not work in an emergency? Just open a valve on the front and leave it leaking so it builds pressure but not so much? I actually called truck pro on the rebuild kit and it was like $110, cheaper than NAPA.

Flamed why? You are absolutely correct. Seems like you could connect the inlet and outlet lines of the dryer together to effect a "get home" workaround.

I would think the line from the Drivers side is pretty big but you could make that little T work off the top and just crimp the other line. I guess that could be a WILE E COYOTE weekend test project.

All you need is an adapter. The number of fitting you want to remove determines which type. You need to remove the line on the right and connect it to the line from the bottom, in the picture. I took the picture facing the rear of the truck.

Unscrew each line at the first fitting and you'll need a Compression to AN adapter.

Or

Unscrew each line and use the next fitting going into the air dryer and you'll need an NPT to NPT adapter. A little more work but a way more common adapter.

The small signal line on the left should not be an issue. If for some reason it is, it can be clamped off. It just deadends at the air dryer. Everything else will work the same as far as the compressor cutting out. It just won't purge and you won't see the small pressure drop after cut-out.

The experts can post the proper sizes. I wasn't ever very good at eyeballing a fitting to determine the size. I can do bolts, but not fittings.

The obligatory disclaimer:

For emergency use only to get you where you out of harms way. You WILL build moisture in the system running like this. Use DOT approved fittings. For offroad use only. Wear gloves, safety glasses, hard hat and flame retardant suit. Don't try this at home.

Seriously though, this is part of the brake system and for emergency use only. I condone nothing and only answer the question that was asked. Use at your own risk.

Guys most of the time you can remove the valve and clean it to get back going. It is not hard they tend to get gunked up from road grime just be careful not to damage the gasket or o-rings and clean and reassemble.

sent from my decrepit fingers

My new dryer will be here tomorrow thanks to member Suprman. Huge Kudos to him, he shipped it before payment was even sent. Perks of being a good boy on here I suppose and always doing right to others! I will install one of the newer variants that hooks up the same way. Since I knew I would be installing that tomorrow, I decided to play around some. No air at all, truck built air just fine and dryer vented with the usual whoosh. Stepped on brakes a few times to drop air pressure and allow compressor to kick in and the expello was venting constantly. Tried the usual push up on it, no luck. Tried oiling it and turning it, still no luck. Disconnected the signal line from the governor and the expello quit leaking. The signal line let go with quite a bit of pressure in it. No air was coming out of the signal line afterward so far as I could tell so something in the base of the dryer might be sticking. Not the most scientific test as I was under a running truck. I tried to spray some PB blaster in the hole to loosen things up but no luck. Even with the signal line disconnected, the truck could build air and cycle the governor at the usual 120 psi. Once I get the new one on, I will take apart the old so I can see if I can be of further help on what failed. I plan to get it operational again to keep as a spare and I also plan to put together an emergency roadside kit. I intend to put together a cap for the signal line elbow and an adapter to bypass the dryer and plumb straight into the wet tank for emergency use only. I do not intend to operate the truck in this fashion for any extended period. I go to GA Rallies which are about 180 miles away from me and would rather be prepared to self recover than pay for a tow truck. Hopefully the new dryer and regular maintenance will make the cost of the emergency kit useless. My Father always taught me, it is better to HAVE IT and not need it than to NEED IT and not have it!!!

Hey Sandcobra, sounds like a good idea to keep a bypass kit in the tool box for emergencies. I just replaced my leaking original style dryer a few weeks ago. I installed a new style like you are getting from Suprman. Great guy to do business with by the way! 2 of the 3 mounting bolts will be too long for the existing plate. A few extra washers did the trick for me. Good luck on the repair. It goes pretty easy. The hardest part was drilling the new mounting holes.

Installed new dryer today and exact same issue as before. Next stop, air governor. I somewhat got upset thinking I had thrown good money out the window for nothing but I thought better of it. I tried to replace the filter on the old one a few months ago and guess what? It is welded on with corrosion and grime. I will chalk it up to basic maintenance and I appreciate the awesome deal I got on the replacement dryer. If you get one from Suprman, it is a pretty easy install. Remove the air lines, unplug heater wires, remove four bolts, catch old dryer, lay new dryer on a piece of cardboard to make a bolt template, drill new bolt holes, install new dryer, transfer fittings as needed. Even the heater wire connection is the same. Only the signal line adapter needed to move from the old dryer to the new dryer base. I will put a new governor on and report my progress. I think I will be happy again and if nothing else, I have gained quite a bit more knowledge of how the air system works on these M939 series trucks.

Howdy sandcobra164,

While you have your spare on the bench it would be a huge help to me (and possibly others) if you could publish the technical info for the fittings required to make an emergency repair bypass kit!

If I crawl under my truck and remove parts to measure them you know something that works just fine today will break or not work tomorrow...

Thanks!

Happy Tails,
tom n tyler

I ran into the same problem. Wouldn't build pressure because the air just came right out of the dryer. I used a glove and a ratchet strap. put a glove over the outlet and strapped it there. It got me home!

sandcobra164 said:

Installed new dryer today and exact same issue as before. Next stop, air governor. I somewhat got upset thinking I had thrown good money out the window for nothing but I thought better of it. I tried to replace the filter on the old one a few months ago and guess what? It is welded on with corrosion and grime. I will chalk it up to basic maintenance and I appreciate the awesome deal I got on the replacement dryer. If you get one from Suprman, it is a pretty easy install. Remove the air lines, unplug heater wires, remove four bolts, catch old dryer, lay new dryer on a piece of cardboard to make a bolt template, drill new bolt holes, install new dryer, transfer fittings as needed. Even the heater wire connection is the same. Only the signal line adapter needed to move from the old dryer to the new dryer base. I will put a new governor on and report my progress. I think I will be happy again and if nothing else, I have gained quite a bit more knowledge of how the air system works on these M939 series trucks.

Click to expand...

You did right by replacing the dryer. It's a better design.

If you haven't ordered a governor yet then PM me your address and I'll send you a new one. I have plenty and they're the military version. I just found that nomination about 20 minutes ago so we'll call it a gift of appreciation

I've had to run the bypass I posted above twice due to bad luck and circumstance. Both times the dryer was damaged beyond repair and left completely inoperable.

The first time an 18 wheeler passed me and lost a recap as he was going by. I saw it come out from under the truck in my mirror and thought I had lost a tire. By the time I got pulled over the spring brakes applied. I lost the whole bottom portion of the dryer to include the purge valve and about the bottom 1/4" of the housing. This was on the first dryer I had installed. I used the spring brake override and the emergency lane to make it to the Home Depot on the other side of the hill. This is where I tried multiple fittings to do the bypass.

Not a month later the second air dryer had the top filter ripped off by a huge water vine under some leaves in the woods. When it ripped the filter off it split the housing. I already had the fittings with me so about 10 minutes later I was back across the field to the house.

Neither time was the dryers fault but I would of been relying on the air in the spring brake tank to get me moving again to a better location. Not a good scenerio. Now I pick up these dryers everytime I come across them since I don't have the best luck. The Purest Air dryers have performed flawlessly for me though no thanks to my own luck.

I bypassed my dryer last year and used the the fittings that were there to do it, as l recall. M931a2

I tried to call SwampDonkey to thank him and tell him he needs to find better friends!!! I only have trucks that get one of a kind problems that are difficult to track down. New air dryer installed, problem still there. New governor installed, problem still there. What was the problem, the braided air line that runs from the governor to the connection beside the transfer case. It had some kind of a blockage that only allowed air to go one way and held it up long enough to create a problem. Cleaned the blockage out in the short term but will likely change the air line out for a long term solution. New governor created yet another issue but can be adjusted. Now it vents at 135 psi. The brakes are very touchy now. I will get it dropped back down to the usual 120 and all will be well!

No problem buddy. Now you have a spare. Those governors came out of a box of FMTV stuff so, like you said, it most likely needs to be adjusted. Unscrew the cap on the end and it can be adjusted with the set screw. Glad you found your problem.

I had the same problem on the 923a2, new dryer, and the valve would pop off, so I adjusted the governor back to 120 and have not had the problem again

I have been an avid lurker of this thread as my air dryer acted up on me last week.

I was driving the 925A2 to the shipper I contracted to bring the truck from Ohio to CA - when all of a sudden my low air warning went off and my primary gauge had deflated to 59 psi. Since I was only a couple miles away from my destination I slowed down and limited any brake use until I arrived.

When I parked, I kept the truck running so I could possible deduce a source of the air loss. I don't have a lot of experience with air brakes but have gained a lot of knowledge reading other owners experiences. Not knowing what was going on I just knew to look, listen and feel for an air leak.

It quickly became apparent that the air was coming from the bottom hole of the air dryer (especially after I took a picture of it and asked everyone, "what IS this?")

My main concern was: What can I do to fix this when I get the truck delivered - since it's not building air pressure and I'd need to get it off the flatbed when it arrived in CA.

The suggestions I received were all great.

1) buy a new dryer (from member Suprman)
2) for a quick fix - attempt to tap the valve closed by inserting a driver in the bottom hole.
3) rebuild the valve
4) clean the valve

While the truck was en route to the left coast I promptly ordered a new air dryer. I then anxiously awaited its arrival - knowing I would attempt the quick fix and hope for the best.

I'm happy to say the truck arrived today and the quick fix worked. I was able to build air and drive the truck off the flat-bed and to the house without incident. I will install the new dryer when it comes but for now, all is good.

Thanks to all for the lessons learned.

Which dryer did you install the factory one or the air purest one?