TM 9-2330-324-14&P
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BRAKE SYSTEM THEORY OF OPERATION
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HYDRAULIC SURGE BRAKE SYSTEM
The M105A3 uses a surge actuator brake system. Surge braking is accomplished by the use of an
actuator located at the trailer coupler. When the towing vehicle slows down or stops, the momentum or
“surge” of the trailer moving toward the towing vehicle automatically applies the brakes.
Unlike the M105A2 or earlier version trailers which used an air-over-hydraulic brakes that engaged only
when the brakes from the towing vehicle were engaged, the M105A3’s brakes are applied whenever its
momentum dictates. The nature of such a system results in a “live” brake action, which means that slight
brake application will occur during normal towing minor disturbances to the trailer, such as bumps, hills,
and curves. This braking will occur at times without any brakes being applied by the towing vehicle. This
could result in higher brake temperatures.
The surge actuator also has an emergency braking breakaway system. In the event of the trailer breaking
away from the towing vehicle, a breakaway cable that is connected to the towing vehicle pulls a breakaway
lever on the surge actuator, which in turn engages the trailer brakes.
The major components of the hydraulic surge brake system are as follows:
1.
Drawbar Coupler—attaches to the towing vehicle pintle hook. The inner casing controls the master
cylinder: when the towing vehicle slows down or stops, the weight of the trailer pushes the drawbar
coupler into the towing vehicle and the trailer brakes are applied; when the towing vehicle goes
forward, the drawbar coupler is pulled and the trailer brakes are released. The drawbar coupler has a
built-in shock absorber to prevent jerky drawbar coupler movement.
2.
Master Cylinder—converts mechanical motion of the drawbar coupler and breakaway lever into
hydraulic pressure. The master cylinder has an oriface and a damper that slows the rate of hydraulic
pressure increase when the towing vehicle backs up. This allows the trailer to be slowly backed up
for short distances on level terrain.
3.
Breakaway Lever—controls the master cylinder in the event of trailer breakaway. When the
breakaway lever is not engaged, the drawbar coupler controls the master cylinder.
4.
Breakaway Cable—is connected to the breakaway lever and attached to the towing vehicle.
4.
Hydraulic Brake Hoses and Tubes—transfer hydraulic pressure from the master cylinder to the
wheel cylinder.
5.
Wheel Cylinder—converts hydraulic pressure into mechanical motion. When the wheel cylinder is
pressurized, it pushes the brakeshoes against the brakedrum.
6.
Brakeshoes—are pushed against the brakedrum by the wheel cylinder. Brakeshoe and brakedrum
friction slows and stops the trailer.
N O T E
The brake system routing diagram is located on page 0002 00-7.
