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M818 Transmission breather leaking air

ROARKY

Purveyor of rusty iron
88
99
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Location
Flemingsburg, KY
Hello! I searched the threads and did not find a thread with this particular issue. I recently acquired a running m818 with title for a fair deal. Once I got it home, I took it for a spin on my field. A few things I noticed right away. 1. There is a constant air leak coming from the transmission breather forward the shift tower. 2. The front axle doesn’t seem to be engaging. There is no air sound when shifting per normal operation. The 2nd issue, I am going to flush the transfercase with diesel and I will also be adding air tool oil to the air lines. The first issue I am not sure on. I went through the TM’s and haven’t discovered anything just yet. I appreciate it!

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Brutacus

Active member
Steel Soldiers Supporter
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Location
Marion, AR.
I had a similar problem when I first got my M818. I thought the poppets were malfunctioning, but it turned out to be the sprag air cylinder on the transfer case. The rubber gaskets on the brass piston had been replaced with O-rings instead of the cup style gasket. It somehow made it seem like air was leaking out of the poppet valves/transmission.

Reverse Sprag, reverse light switch 1.JPG
 

ROARKY

Purveyor of rusty iron
88
99
33
Location
Flemingsburg, KY
The mystery deepens. Other than finding my steering wheel broken?!?! I discovered the air vent port, located by the two (valves) on the side of the transmission, is plugged. From photos of other trucks found on here, there’s were open, no plug. My hunch is to remove this plug and see how it affects the air leak. I can see though that I need to replace the large rubber air lines as well. Anyone know a kit for these?

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MyothersanM1

19K M1 Armor Crewman
Steel Soldiers Supporter
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68
Location
Culver City, CA
The mystery deepens. Other than finding my steering wheel broken?!?! I discovered the air vent port, located by the two (valves) on the side of the transmission, is plugged. From photos of other trucks found on here, there’s were open, no plug. My hunch is to remove this plug and see how it affects the air leak. I can see though that I need to replace the large rubber air lines as well. Anyone know a kit for these?
Oh! No bueno!

As far as the rubber lines, I replaced mine with DOT push-to-connect brass fittings and 3/8" DOT nylon air brake tubing (SAE J844 Type B).
SDC12047.JPGSDC12048.JPG
 

ROARKY

Purveyor of rusty iron
88
99
33
Location
Flemingsburg, KY
I unplugged the pipe cap that the previous owner had installed on the poppet valve vent. The truck now audibly expels air when shifting from first and reverse. It may be nothing but, I don’t recall the breather on the transmission constantly venting air on my previous m818. Planning on redoing all air lines with new along with fuel lines because the truck surges from around 500rpm and then drops to around 400 rpm. I am pretty sure it is sucking air. Gonna flush the transfer case and bypass the selector valve for the fuel tanks. Once I have this issue figured out, I’m planning on running the fuel fuel line mod and updating filters.
 

ROARKY

Purveyor of rusty iron
88
99
33
Location
Flemingsburg, KY
Alright, making progress. Pulled the dog house cover for easy of access (will also be out of the way for the duel fuel line mod). I drained the air tanks after running the truck for awhile. There was quite a bit of water! I’m not sure they had been drained in awhile. Good news first, the forward transmission breather is no longer venting continuously! When I unplugged the poppet vent hole, the pressure now vents through it when in neutral and forward gears. When I select reverse, it stops venting. When going from reverse to neutral/first, the vent continues to vent air until I select reverse again. As you may imagineX this causes the trucks low air pressure buzzer to come on after a few minutes. A bump of the throttle causes it to go back off and then gradually it will come back on.

Diagnosis:
Air leak: The poppet valves, maybe just the forward selector, is sticking, may have bad o-rings.
Fuel surge: I am almost certain at this point that I am drawing air.

Treatment:
Air: I ran some seafoam deep creep through the airlines to see if I can free up the valves. It says on the cab that it can be used for air tools so I figured let’s give it a shot. I also plan to drain the transfer case, run diesel in it while empty and drain until it looks good. I plan on replacing any questionable air lines while I’m there. If it does not straighten up, I’ll plan on doing both valves with a new o-ring kit I found on the introvertednet.
Fuel: I am bypassing the tank selector valve. I plan to run a transfer pump from the passenger tank to the drivers tank and only running the truck from the drivers tank. I believe a small 24v in-line fuel pump will be adequate. I plan on running all new fuel lines from the tanks forward of the selector valve. I also plan to replace any plastic lines I find that are questionable. While I’m doing this, I then plan to do the fuel fuel line mod.

Truck will be getting all new filters, oil change and some cosmetic work since it has a minor rust issue.

Contingency plan: if the air issues for the transfer case are not solved with the poppet valve rebuild, I will most likely source a t1138 and install it in place of the t138. I have a spare t138 but I think I would be happier with the lower rpm since I’m running stock 11.00x20. I do plan to upgrade to 11r24.5’s eventually. I will only use the truck off the interstate and local, mostly using it as a big farm truck. I might eventually use it to pull an rv but again just to the local lake. Any farther, I would want to get a more modern rig.

I am sure I don’t have to explain this to y’all, but I mess with these trucks mainly because my dad was in transportation in the army from 1978-1981 I believe. He still drives a truck and loves seeing them still. It would be cool if someone served during that time frame. He was 2/7 Calvary when he got out. Served a lot of his time in Wurzburg, Germany.

Will update as I progress
 

MyothersanM1

19K M1 Armor Crewman
Steel Soldiers Supporter
1,957
418
68
Location
Culver City, CA
Fuel: I am bypassing the tank selector valve. I plan to run a transfer pump from the passenger tank to the drivers tank and only running the truck from the drivers tank. I believe a small 24v in-line fuel pump will be adequate. I plan on running all new fuel lines from the tanks forward of the selector valve.
The transfer pump is a smart move. I did the same mod.
I also replaced all my supply and return fuel lines from the left tank with AN fittings and braided nylon/stainless steel fuel hose.
 

US6x4

Well-known member
Steel Soldiers Supporter
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2,214
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Location
Wenatchee, WA
When I select reverse, it stops venting. When going from reverse to neutral/first, the vent continues to vent air until I select reverse again. As you may imagine, this causes the truck's low air pressure buzzer to come on after a few minutes.
my m813 still has a similar issue but maybe on the opposite side of the poppet valve. After reversing into my parking spot the T-case is usually venting air out of its breather cap when I shift from reverse to neutral to park. I have to cycle the shifter to reverse and back to neutral again 1, 2, or 3 times to get that air to stop venting.

I don't know what it is exactly, but exercising the valve convinces the air leak to stop. I should get a rebuild kit for the valve and the sprag just to have on hand in case it all quits working.
 

ROARKY

Purveyor of rusty iron
88
99
33
Location
Flemingsburg, KY
my m813 still has a similar issue but maybe on the opposite side of the poppet valve. After reversing into my parking spot the T-case is usually venting air out of its breather cap when I shift from reverse to neutral to park. I have to cycle the shifter to reverse and back to neutral again 1, 2, or 3 times to get that air to stop venting.

I don't know what it is exactly, but exercising the valve convinces the air leak to stop. I should get a rebuild kit for the valve and the sprag just to have on hand in case it all quits working.
sounds like my issue except it continues to vent. I worked the shifter back and forth several times, enough to cause the air buzzer to come on. I’ve read on here to flush the transfer case with diesel. Going to try that. Got a brake issue to fix first.
 

gimpyrobb

dumpsterlandingfromorbit!
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Location
Cincy Ohio
So those valves on the trans move the piston on the T-case to turn on forward or reverse Sprague. The valves can be taken apart and cleaned/rebuilt.
 

Floridianson

Well-known member
Steel Soldiers Supporter
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Location
Interlachen Fl.
Your sprag cylinder need R&R. As someone said no O rings but TM calls them preformed packing. You can wait on buying parts if you want because you might just need the two preformed packing and two new copper crush rings. Or you might need the kit that has new cylinder to if yours is rusted out real bad. To remove the cylinder you need to air up the truck to full air and put the Transmission in reverse. Now leave it in reverse and shut off truck. Drain the air from all air tanks. Now you can bend over the retainers on the four bolts that hold on the cylinder cap and slip the cylinder off the piston. If I remember 5/8 deep well socket to remove piston retainer nut. Then piston should just pull off the sprag rod going into the Trans. If not pull read hard or use nylon strap wrench to try and unscrew it. Sometimes they come off easy sometimes hard but it does just pull off. If cylinder is in good shape and you just need packing then we oil up the cylinder and install the piston in the cylinder and install it as one piece. Then packing into the cylinder install copper washers and nut. The piston will go in the cylinder easy for one of the performed and you just use your finger nail to help the the other end to slip into the cylinder.
 
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Floridianson

Well-known member
Steel Soldiers Supporter
7,409
2,501
113
Location
Interlachen Fl.
my m813 still has a similar issue but maybe on the opposite side of the poppet valve. After reversing into my parking spot the T-case is usually venting air out of its breather cap when I shift from reverse to neutral to park. I have to cycle the shifter to reverse and back to neutral again 1, 2, or 3 times to get that air to stop venting.

I don't know what it is exactly, but exercising the valve convinces the air leak to stop. I should get a rebuild kit for the valve and the sprag just to have on hand in case it all quits working.
When you have air leaking out of the transfer case vent the sprag shaft seal to transfer case is leaking. Some rebuild kits come with the new seal that we can replace when we R&R the sprag piston and cylinder.
 
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