how to plumb a civilian air braked truck to flat tow with brakes like military does

[466Navastar]

Im looking at an all wheel drive 1996 autocar ACL that has a bad transmission....wonder if any of you fellas can give me a thumbnail explanation of how one would go about plumbing this truck to utilize its brakes while being flat towed by another air brake equipped truck - like is commonly done with the green iron...... thanks

 

[wreckerman893]

Since commercial tow companies that do heavy duty work have to do this all the time I'd ask an expert.

 

[466Navastar]

maybe what I should have asked is ......how are the military trucks plumbed ?

knowledge doesn't hurt anyone - might help to know so I can judge how expert the professionals plan is when I do ask....or maybe if its straight forward I might be able to do it myself and save more than a few bucks

thanks in advance for any info - Im not looking for a book

 

[wcuhillbilly]

Im still learing myself, but I know that some run a line or set of lines to the glad hands on the back of the cab where the civy trailer would hook to the Civy truck and basically reverse air the system, while minding the valves on the dash.
Other versions and more common is to tap into one of the front air cans on the front axle, (if this axle is up no need for brakes) and revers supply the air system with the supply to the front air can.
If your setting up a system.... an easy way to supply air is to put an air fitting like a regular air chuck for an air hose on one of the tanks, or a small line coming from the air tanks to an easy location on the frame, This will allow multiple functions, outside of suppling the air system by slaving on another truck, it will also allow you to run an air hose or air tools to work on your own truck.. Side note,,, a foam ear plug inserted in the female end of the air fitting will keep the dirt out and save you time cleaning it when your up to you arse in the muck needing the air line.

 

[466Navastar]

thank You wcuhillbilly

 

[wcuhillbilly]

Thanks,,,, side note, my reply will allow you to release the brakes, but I dont know as of now how to re engage the brakes while towing as in the way the military trucks will. Im sure it can be done but I havent scratched my head and arse long enough to figure it out... mine is air over hydro so a release of ( i dont remember which) supply or emergency will activate the brakes and then build up of the dumped air will release.... the Civy rigs with the dash valves is a little more tricky, as the yellow valve will dump tractor air, thus a parking brake for the tractor, same thing is accomplished by the pedal, but you can pop both vavles by variing air pressures or by dumping air on the trailer/red button side... which if I recal is our "emergency" side of the plumbing.

I need to put a red button on mine as I tow an air braked trailer and it gets to be a pain to pull up and Mico brake lock the truck then walk around and shut off the ball valve on the Red/passenger side then blow the glad hand and take a dust bath. If the Red button was installed I could dump that Emergency side and the truck and the trailer would act as a parking brake for me to disconnect the air lines to the trailer without taking a dust bath.


Some of the following posts in referance to my comments above lead me to believe that the use of the word "reverse" has been taken a little out of my original contexts. I simply meant to supply the towed truck with air. I did not mean that that supply would be used for anything else except relaease of the brakes. I stated that I did not know how to utilize the air for actuation of the brakes on the towed truck... Im still learning,,, "MyothersanM1" has a good write up about how he plumbed his 818 for modern air brakes and if someone needs a full explination I suggest checking out his air brake conversion here on SS.

 

[dmetalmiki]

You can not work the air supply in "reverse" to apply the brakes. Military trucks have one way valves on the outlets (front service) to prevent air loss. The emergeany lines (front) accept air to charge the system. The service lines (rear) give apply air, and have double check valves. Emergeancy air is charged air in the tanks. Have a read up in T.M.9-811. , T.M.9-8022. or, T.M.9-2320-211-35. my 1/2 penne'th.

 

[sweetk30]

also civi truck might need air bags pumped up and air to keep them up . if its air ride .

and make sure before you even move the truck pull the drive shaft .

my boss tows big stuff with his 35ton kw and some of the storys of trucks these days with auto trans or even some man trans you move them NOT under there own power = dead trans & big money fix .

 

[painter paul]

Civi trucks generally can't be aired up in reverse because of one way check valves in the air tanks. They are usually aired up by going into the wet tank(first tank back from the air compressor) or going into the line that is the outlet from the compressor. Google a tool called a Brake Buddy which is an air cylinder tool that clamps between the steering wheel and the air Brake Brake pedal. It's hooked to an service side air line from the tow truck. When the tow truck's Brake pedal is pushed, air flows thru the air line, extends the air cylinder tool which pushes down on the towed trucks Brake pedal, applying its air brakes. They cost around $600. Lots of air Brake trucks are towed with only one air line to it to release its brakes. However if the towed truck is loaded the brake buddy is highly recommended. Especially if the tow truck isn't big enough to be able to stop much more than its own weight.

 

[sweetk30]

Civi trucks generally can't be aired up in reverse because of one way check valves in the air tanks. They are usually aired up by going into the wet tank(first tank back from the air compressor) or going into the line that is the outlet from the compressor. Google a tool called a Brake Buddy which is an air cylinder tool that clamps between the steering wheel and the air Brake Brake pedal. It's hooked to an service side air line from the tow truck. When the tow truck's Brake pedal is pushed, air flows thru the air line, extends the air cylinder tool which pushes down on the towed trucks Brake pedal, applying its air brakes. They cost around $600. Lots of air Brake trucks are towed with only one air line to it to release its brakes. However if the towed truck is loaded the brake buddy is highly recommended. Especially if the tow truck isn't big enough to be able to stop much more than its own weight.

and this info is only good if your towing with the front axle off the ground and steering wheel locked / tied off to not move and kick the brake buddy out .

 

[MyothersanM1]

I recently retrofitted my M818 with full air brakes; s-cams on the rear and M939 type wedges on the front.

I did plumb the the front gladhands to supply and actuate the brakes. The emergency gladhand was plumbed directly to the wet tank which supplies air for everything on the truck including the primary and secondary tanks. There is a check valve right behind the gladhand allowing air to flow into the truck but not out that gladhand. Pretty straight forward.

The service line is a little more complicated. The front axle proportioning valve is fitted with a double check valve on its supply port. The front service gladhand connects to one side of the check valve with a tee fitting and the on board secondary actuation line from the treadle valve to the other. The on board secondary line will override the gladhand line through the check valve in normal operation and vice versa.

Now the tee fitting will carry the gladhand service line to a second double check valve. At this check valve, the primary (rear) actuation will be determined. The primary circuit from the treadle valve is connected to one side and the front service gladhand to the other. The line to the rear is connected to the center. The primary circuit will override the front gladhand in normal operation.

My truck has an R-14 relay valve for the rear axles which is constantly charged through the primary tank and is actuated from the line leading from the second double check valve. Sounds complicated but it is rather simple.

 

[quickfarms]

When towing a tractor trailer. The tractor is usually just aired up to release the maxi's and air up the air bags. A service and supply, emergency, line is run to the trailer.

If you need the truck air brakes to function there is an existing port on one of the valves for that, just remove the plug and attach an air line. At least this is how it is on my trucks.

 

[466Navastar]

thanks to all that shared their thoughts