Lesson 2/Learning Event 1
The master cylinder used with airhydraulic brakes is like the one
described in the lesson on hydraulic brakes. In straight hydraulic
brake systems, the master cylinder receives the initial mechanical
force from the pedal linkage, changes it to hydraulic pressure, and
sends the brake fluid under pressure directly to the wheel cylinders.
In airhydraulic brakes, the master cylinder sends brake fluid under
pressure to an airhydraulic cylinder before it goes to the wheel
cylinders. On all military designed vehicles, the master cylinder
has a vent fitting at the top of the reservoir for connecting a vent
line to the vent system of the vehicle. This prevents water from
entering the master cylinder through the vent during fording
operations. The special drilled bolt and fitting installed in the
filler cap of the master cylinder serves this purpose.
The airhydraulic cylinder is put into operation by the hydraulic
pressure from the master cylinder. It uses compressed air to boost
the hydraulic pressure from the master cylinder. The Army uses more
than one model of airhydraulic cylinders; all models contain the
same major units and operate on the same principles. They are made
up of three major units in one assembly. The units are the control
unit, power cylinder, and slave cylinder. The units of the M809
series vehicles consist of an air valve, air cylinder, hydraulic
cylinder, and piston.
The control unit contains a control valve (relay) piston, which is
hydraulically operated by brake fluid from the master cylinder, and a
diaphragm or compensator assembly, which is operated by pressure
differences between brake fluid and air and spring pressure. A
return spring holds the hydraulic relay piston and diaphragm assembly
in the released position when there is no hydraulic pressure. Two
air poppet valves, assembled on one stem, control the air pressure
flowing into and out of the power cylinder. The poppet valves are
normally held in the released position by the poppet return spring.