1800 SRM 951
Description and Operation
General
This manual has description, operation, and repair
procedures for components of the brake system.
The main parts of the system are the hydraulic
pump with charge valve, two brake pedals, brake
pedal valve, two accumulators, oil cooler, two wet
disc brakes, park brake, and park brake valve. See
Figure 1, Figure 2, and Figure 8.
Description and Operation
The hydraulic pump (see Figure 1 and Figure 2)
supplies the oil to operate the brakes and cool the
brakes and the parking brake. Oil pressure for the
brakes is maintained between 14.8 and 18.5 MPa
(2146.5 and 2683 psi).
The accumulators have a
precharge pressure of 9.0 ±0.5 MPa (1305 ±72.5 psi).
When the brake pedal is operated, the brake treadle
allows oil to flow to the brake pistons. There are two
oil pressure checkpoints near the brake treadle. One
detects pressure and is used for the brake lights and
the other one detects insufficient pressure and is
used for indicating/signaling at the instrument panel
and instruments. When the brake pedal is released,
oil pressure in the brake cylinders is relieved to the
hydraulic tank. Brakes can be bled at the manifold.
When pressure in the system reaches 18.5 MPa
(2683 psi), the charge valve switches to provide pres-
surized oil for the cooling of the brakes. A return
line carries the cooling oil back to the hydraulic tank
via a filter. A filter bypass is also provided. When
pressure drops below 14.8 MPa (2146.5 psi), the
charge valve switches back to provide pressurized oil
to the brake system. The parking brake is operated
via the parking brake valve.
SERVICE BRAKES
There is an oil-cooled brake assembly at each drive
wheel as shown in Figure 3. There are discs that ro-
tate with the wheel hub and stationary discs that are
locked to the brake housing. The discs are assem-
bled so that there is a friction (rotating) disc between
each stationary disc. A piston, moved by hydraulic
pressure, pushes the discs together for the braking
action. A separate oil cooling system circulates oil
through the sealed case to cool the discs. The brake
pedal activates the brake treadle and controls the
flow of oil to the service brakes. Pushing the inch-
ing pedal will at first control the declutch action and
further pushing will activate the brakes due to the
mechanical coupling to the brake pedal.
PARKING BRAKE
The parking brake system uses a spring-applied
caliper and is installed at the rear of the differen-
tial. Under normal conditions, the red knob on the
instrument panel is pushed in, and an electrical
signal triggers the parking brake valve. Oil pressure
compresses the spring and the parking brake is re-
leased. The parking brake is applied by pulling out
the red knob on the instrument panel. The parking
brake valve position changes, and oil pressure in the
parking brake cylinder is relieved. This causes the
springs to apply the brakes for parking. The spring
automatically applies the brake if the system loses
oil pressure. The minimum system oil pressure is
14.8 MPa (2146.5 psi).
OIL COOLER CIRCUIT
The oil for cooling of the brakes is cooled by the oil
cooler. See Figure 4. In the event that measured oil
temperature is above 97 C (206.6
F)
, a cooling fan is
activated. When the temperature of the oil drops be-
low 92 C (197.6
F)
the fan is switched off. The cooling
system oil pressure is 0.1 MPa (14.5 psi).
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