TROUBLESHOOTING THE HMMWV - OD1616 - LESSON 1/TASK 1
The dual master cylinder contains two brake circuits that are
separated hydraulically.
The individual brake systems may be
designed to divide the system front to rear, diagonally, or in
various other fashions. In this case, the system is divided front to
rear.
If a brake fluid leak develops in one circuit, the other
circuit still provides emergency stopping capability.
As the brake
pedal is depressed under normal operating conditions, it forces the
primary piston forward to cover the primary compensating port.
At
this time, the primary chamber is sealed and direct hydraulic
pressure is transmitted to the secondary piston. As the brake pedal
continues to travel, the secondary piston covers the compensating
port.
Further application of the brake pedal develops the pressure
required to apply the brake components.
Should a leak develop in the primary circuit, the brake system would
not be rendered useless.
During the application of the brakes, the
primary piston would continue to move forward, unable to build up
pressure due to the malfunction.
Approximately halfway through its
maximum stroke, the primary piston contacts the secondary piston.
Further application of the brake would force the secondary piston
forward to develop pressure in the secondary system, which would
allow for braking action to take place in two wheels.
Should the
secondary circuit fail, braking for the other two wheels would still
be available.
The primary piston would move forward and cover the
primary compensating port as before.
Because of the rupture in the
secondary circuit, the secondary or floating piston would be moved to
its extreme stop by the force of the return spring.
Further
application of the brake would develop enough pressure in the primary
circuit to apply the brakes connected to this circuit, thereby
allowing the vehicle to maintain some stopping ability.
(2) Brake Lines.
The brake lines used on the HMMWV are of 3/16
inch seamless steel tubing with flared fittings.
Flexible hoses
connect the brake lines to the wheel calipers.
(3) Brake Fluid. In paragraph 6c(1) on the previous page, it was
stated that the master cylinder contains .69 pints of BFS.
The
entire system, master cylinder, lines, calipers, etc. contains 1.2
pints of BFS.
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