TROUBLESHOOTING THE HMMWV - OD1616 - LESSON 1/TASK 1
manually actuated, hydraulically assisted, and operated.
two brake systems in use on the HMMWV.
One is the service brakes,
and the other is the parking brakes.
Service Brakes. The service brake system consists of the brake
pedal, pushrod, hydro-boost, master cylinder, combination valve,
When the driver steps on the brake pedal, the pedal force is
transferred through the bellcrank assembly, directed into the hydro-
boost unit, where it is hydraulically boosted and passed on to the
brake calipers. The calipers close on the rotor providing a stopping
The braking system is split front and rear so that if a
hydraulic failure occurs to either the front or the rear brakes, the
vehicle may still be stopped with the other two brakes.
The parking brake is a lever operated brake
that functions through a cable linkage.
Its primary purpose is to
hold the vehicle stationary.
A secondary purpose of the parking
brake system is to stop the vehicle in the event that there is a
complete brake failure.
The parking brake consists of a parking brake lever, cable linkage,
mechanically operated brake shoes and a rotor mounted to the rear
differential between the drive shaft.
When the driver pulls on the
parking brake lever, the cable linkage squeezes the brake shoes on
the parking brake rotor.
Components of the Service Brake System
. In the paragraphs that
follow, the components that make up the service brake system will be
The master cylinder has two fluid outlet ports, a
piston bore, and two hydraulic piston assemblies.
assemblies are located in the piston bore and are operated in tandem
by a pushrod.
The master cylinder on the HMMWV holds .69 pints of
brake fluid silicone (BFS).