Military vs. Civilian Air Brake Systems

[Memphisdoug]

Do the air brakes on a military 5th wheel semi trailer work like a civilian system (air pressure disengages the brakes) or like the system for a M105 trailer (air pressure applies the brakes). Here is why I am asking - If I make a 5th wheel dolly trailer out of a M200 trailer, can I use it to tow a military 5th wheel trailer behind my duece and have compatible brake systems? In other words - will the airlines of the military trailer plug into the back of my duece and operate the trailer brakes correctly.

I am assuming the same dolly could not be used to tow a civilian type fifth wheel trailer with air brakes?

Thanks for any help.

 

[tmbrwolf]

The military semi trailers may have either system, most earlier semitrailers had air / hydraulic similar to the 2 1/2 & 5 ton trucks later ones (like the M872) went with an air only system. Since you are going to use the air connections on the duece to apply the brakes compatability shouldn't be an issue. Yes it will I am assuming the same dolly could not be used to tow a civilian type fifth wheel trailer with air brakes? Maybe yes maybe no! It could tow the trailer with straight air brakes either military or civvy but it may not apply the brakes correctly, on an air brake equipped semi tractor the air pressure is applied proportional to pedal pressure across the whole system, on the air / hydraulic equipped truck it may or may not provide enough pressure to apply the correct amount of pressure for proper braking or it could apply too much pressure. We did have issues with the older military trailers when we were issued the M900 series tractors which are straight air brake pulling the M127 trailers with air / hydraulic brakes, the problem was that the tractor brakes are much quicker to apply and the trailer brakes were weak compared to the tractor, not much of a problem on dry pavement but add a little rain...... the trailers would try and pass the truck, similarly when pulling an air brake equipped trailer with and older 5 ton tractor (M81 the trailer brakes would apply before the trucks brakes would.

 

[dm22630]

I thought the only difference between military semi trailer & civy ones were....

Military trailers do not have brakes engaged when there is zero air pressure & civy trailers brakes are locked until air pressure reaches a certain level?

 

[kc5mzd]

The "new" civy trailers release the parking brake at around 60psi. The "old" civy trailers didn't have the parking brakes. I forgot what year the DOT started requiring the parking brakes I think it was in the '80s. The civy parking brake works off the main air system not the braking pressure. The M35A2 has 2 glad hands one for the main air pressure that will fill a tank to about 90 -120 psi and release the parking brake, the other is for the brakes. I have a couple of M105A2 trailers and some are setup like civy tractor trailers with 2 glad hands. The others only have 1 glad hand that directly applies the brakes with no tank. Both work fine. You just have to make sure that if there is a tank that you hook up the second air hose. If you don’t the trailer brakes won't work.
A M35A2 should work with the new type civy system without any problem. You just have to wait for the low air buzzer to go off before the parking brake will release.
As far as stopping goes I might try to adjust the trailer brakes to try to compensate for any differences in the pressure as tmbrwolf was describing but I don't know how much luck you might have.

 

[m16ty]

civilain trailers with air brakes have always had parking brakes ( in the last 50 or 60 years anyway). What they haven't had is spring brakes that became manditory sometime in the 80s. A trailer without spring brakes still has a parking brake but when the air leaks off the trailer air tanks the brakes will release. A trailer with spring brakes has a big spring inside the brake chamber that sets the brakes when parked so they won't leak off.

I'm pretty sure a deuce will work with civilain air brakes fine. You've got an emergency and a service line on the back of the deuce which is just like all civilain trucks.

Edit- After going back and re-reading timberwolf's post he could be right about them being not "balanced" in braking.