How do trailer brakes work?

On the M105/M200A1 and similiar trailers, what activates the brakes when you press the pedal in the cab, the air or the electric? I know that the emergency line if disconnected will activate the brakes but what about normal braking?

Trying to figure out a way to make a trailer that uses the air glad hands like a military one. I tried to customize a M200A1 trailer to haul my 88 Jeep Comanche but was thinking just custom make the whole thing this time around.

Air activates the brakes through an airpack of sorts on the trailer.

Basically there are 2 different braking systems. One with a single hose and the later ones with 2 hoses. The single hose one is a very simple system. There is a master cylinder with an air chamber on it. When you appy truck brakes, the air pack meters air back to the air chamber on the trailer and it applys the brakes. The newer ones have a relay valve and an air storage tank on the trailer in addition to the master with the air chamber. On this model, the metered air signal is routed to the relay valve which in turn, sends air to the chamber and applys the brakes. The relay valve provides quicker application of the brakes but what it does mostly is contain a break away feature. If the relay senses a loos of air pressure from any means, it uses the air stored in the tanks and applys the brakes on the trailer. If you ever have a runaway trailer or break an air hose, you will see this first hand.

trailer brakes

Here is the info on the M200A1 from TM 9-2330-205-14&P. There is another trailer you might want to check out. M353 is a generator trailer but it has a single trailing arm axle so your center of gravity is lower. Wayne


Relay Valve - Controls the braking system of the trailer. Based on the air pressure signals received from the towing vehicle, it will apply or release the service brakes or it will initiate an emergency brake application.