T-138 sprag question.

TomTill84

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I'm new. Joined site to help me figure out what truck I would like to own. I've read through many and I mean many posts on here and have learned a lot. Thanks to all of you. I'm leaning towards a M 818, and adding a M35 bed. I know people hate the sprag T case. Was looking at the TM for it and noticed there are 2 air lines going to it. One I have no idea what it's for or what it does. The other goes to the sprag cylinder? I 🤔. Anyways, if that air line is disconnected will it keep the sprag from engaging the front axle?
 

98G

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I've had a couple of M818s. I can't imagine why you'd want to disable the front axle. (I *can* see why you'd want it only on demand)

In contrast to the sprag tcase on the deuce, the T138 is robust and works well in my experience. I've had an M818 buried to the axles and got it out under its own power.

I like the idea of a truck tractor length wheelbase with a 12' bed.
 

TomTill84

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I don't want to disingage it all the way. I would put on some type of valve in the cab to turn the air on and off in case I need it. So disconnecting that air line would keep it form ingaging the front???? Yep, seen a pic of a M818 with the 12 foot bed, liked the way it looked!
 

Floridianson

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Yep some guys have done the air control mod. Disconnect the rubber air line that comes off the passenger frame rail that runs to the poppet valve and run a new line into the cab to a air control switch. Then return air feed line from switch to poppet valve. Both air lines control the sprag cylinder piston. One line is on the face plate of the cylinder and that one is for reverse sprag. The other air line goes into the transfer case and supplys air to the cylinder and is for forward sprag with poppet valve controlling everything.
 

Floridianson

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As said I never tried this mod so I do not know if after you cut off the air if the line after the switch could hole enough air to hold sprag in forward or reverse. You can try and see if it does hold air by just switching from reverse to neutral with air cut off a couple of times. Sometimes you can hear the air releasing from the poppet or even feel when the piston slams into the case. We want the sprag position in a so called neutral. This happens because if we look at the sprag piston on both ends are caps and springs. With air pressure on one side or the other of the piston it is enough to hold the spring collapsed and sprag engaged. No air then the sprag is not engaged as spring is expanded enough to hold a disengagement.
Also been a long time but believe both the block on the frame rail and poppet valve were NPT so here you could use Synflex air line and correct fittings and easy short lines do a floor mount switch maybe between the seats. Good thing Synflex easy to run so you could have switch on dash if you want to run lines that far.
 
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TomTill84

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One would think that no air supply would put it in a neutral position. Say the truck broke down and had to be towed, if it was towed with the front up the steer tires would be turning as it was being towed. The speed offset would engage the sprag. My 2 cents.
 

Floridianson

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Yea you would figure that it was just something I thought might happen ( over thinking). I just posted that in case there is some way that the piston was still engaged. As said I never did the mod but as said you could see if for some reason it could happen with the truck shut off but aired up plus air to poppet on then and air to sprag cut off then try and see by switching from reverse to neutral a couple of times and hear if any air is being released from poppet. If you do the mod and try it just report back and tell us if the results. I could be over thinking it but stranger things have happened.
 

Floridianson

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You would think all air would leak out with the rubber seals and the way the rubber packing is designed on the sprag cylinder piston /poppet. Not real great but they work well till they go bad. When you go from forward to reverse the air you hear coming out if the poppet is the air being let loose from the forward side of the piston allowing the piston to go into reverse position on piston. As said over thinking it but sometimes over thinking is ok. So look at it this way air seals hold real good in both poppet and sprag cylinder. If you shut off the air while the sprag was engage it will stay engaged till the air leaks out of seals or we release it by going from reverse to neutral a couple of times. Also we do not need to go into first gear to engage forward sprag as it happens as soon as it goes in to neutral.
 
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Lovetofix

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Floridianson you are right on in your thinking. I have done this mod on my M817 and you need a valve that will vent the line going to the poppet valve at the same time it closes off the air supply, otherwise there is still enough air pressure to actuate the piston left in the line.
The perfect valve for this mod is from a deuce and a half with the air engaged front axle. It even has the label and telltale light to tell you if in is “In” or “Out.” Sorry I don’t have the NSN number for that switch/valve.
 

Floridianson

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Also a for air leaks we have the transfer case seal that holds the cylinder air pressure for forward sprag and if it leaks good then in would go into a neutral position from just sitting truck off. You can tell when you transfer case sprag seal is leaking when in forward sprag and you get air coming out the transfer case breather.
Also Lovetofix name is James to you sir.
 

Brutacus

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If you add a duece bed on the M818, you can remove the tire carrie/tool box and have plenty of room for the 12 ft. bed. If you keep the tire carrier/tool box, you might have to add a frame extension for the 12 ft. bed, or shorten the bed. If you look at the frame behind the rear wheels you'll see it's a different color. That's an extension the previous owner added so they can keep the tire carrier/tool box.

GEDC0022.JPG
 

Floridianson

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Floridianson you are right on in your thinking. I have done this mod on my M817 and you need a valve that will vent the line going to the poppet valve at the same time it closes off the air supply, otherwise there is still enough air pressure to actuate the piston left in the line.
The perfect valve for this mod is from a deuce and a half with the air engaged front axle. It even has the label and telltale light to tell you if in is “In” or “Out.” Sorry I don’t have the NSN number for that switch/valve.
Since we need a air control valve that bleeds off the air on the poppet side like you said the Deuce air switch wonder if a lift axle air valve switch. They do the same thing and bleed off the air in the line when shut off. O well OP has his answers.
 

Floridianson

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One would think that no air supply would put it in a neutral position. Say the truck broke down and had to be towed, if it was towed with the front up the steer tires would be turning as it was being towed. The speed offset would engage the sprag. My 2 cents.
Correct me if Iam wrong as forgetting everything. If we just hook up service air that will not fill the air tanks ( remember correct me if I am wrong ) then drain the air tanks if they are holding air and check to see if sprag cylinder / poppets are holding it's pressure. As said reverse to neutral a couple of times would release any air. Then the sprag would be in it's neutral position ready for lift tow. By just hooking up just service air then we will still have the towed truck brakes working. If I remember if we hooked up emergency air that would fill air tanks and if the truck was in neutral then it would be in forward sprag. So no emergency air needed.
 
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