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The Komatsu truck is equipped with an all-hydraulic
actuated wet disc service brake system. The brake
system utilizes Type C-4 hydraulic oil provided by the
brake/steering pump from the main hydraulic tank for
brake application. Brake disc cooling during truck op-
eration is provided by the hoist circuit pump through
the hoist valve. A disc type parking brake, located in
the rear axle housing, is attached to each wheel motor.
The fundamental function of the brake system is to
provide an operator the control he needs to stop the
truck in either a slow modulating fashion or in as short
a distance as reasonably possible.
Outlined below are the functions that Komatsu Division
feels are necessary for safe truck operation:
• Warn the operator as soon as practical of a seri-
ous or potentially serious loss of brake pressure
so proper action can be taken to stop the truck
before the secondary system is exhausted of
power.
• Provide secondary brake circuits such that any
single failure leaves the truck with sufficient stop-
ping power.
• Automatically apply service brakes if low pres-
sure warnings are ignored and pressures con-
tinue to decrease.
• Wheel brake lock to relieve the operator from
holding the brake pedal while at the dump or
shovel.
• Spring applied park brake for holding, not stop-
ping, the truck during periods other than loading
or dumping.
• Brake system that is easy to diagnose and per-
form necessary service.
The following brake circuit description should be used
in conjunction with the hydraulic brake system sche-
matic, refer to Section "R".
The brake system consists of several major valve com-
ponents; the foot operated dual circuit treadle valve,
hydraulically operated dual relay valves, and the brake
manifold. The dual circuit treadle valve is the only
component located in the operator's cab. The remain-
der of the system, including the dual relay valve for the
front brakes, brake manifold, two accumulators, and
electrical components, are located in a weatherproof
J02027
BRAKE CIRCUIT
cabinet (Figure 2-1) behind the cab. The hydraulic
components cabinet is easily accessible for brake sys-
tem diagnostic and service work.
The dual relay valve for the rear brakes is located in the
rear axle housing. Two additional accumulators are
mounted on the frame rail, behind the right front tire.
The brake manifold contains dual circuit isolation
check valves, accumulator bleed down valves, and
valves for brake lock, park brake and automatic apply
functions. All of these components are screw-in car-
tridge type valves.
In the Komatsu truck, there are two independent
means of brake actuation by the operator; the service
brake pedal and brake lock switch. Additionally, the
brakes will apply automatically if the brake system
supply drops below a predetermined pressure.
SERVICE BRAKE CIRCUIT
This portion of the system provides the operator the
precise control he needs to modulate (feather) brake
pressure to slowly stop the truck or develop full brake
effort to stop as quickly as possible. The heart of this
circuit is the treadle operated, dual circuit brake valve.
This valve enables the operator to control the relatively
high pressure energy within the brake accumulators
directed to the brakes. There are two valves in the dual
brake valve. One supplies pressure to a dual relay valve
to provide apply pressure for the brakes on the front
axle. The other supplies pressure to a dual relay valve
to provide apply pressure for the brakes on the rear
axle.
As the brake pedal is depressed, each valve within the
dual circuit brake valve simultaneously delivers fluid
from its respective accumulators to the dual relay
valves which deliver fluid to the wheel brakes at a
pressure proportional to both pedal position and force.
The further the pedal is depressed, the higher the brake
force, giving a very positive feel of control.
Brake accumulators have two functions; storing en-
ergy for reserve braking in the event of a failure and,
provide rapid oil flow for good brake response.
Depression of the brake pedal also actuates the stop
light pressure switch, which in turn actuates stop and
service brake indicator lights and propulsion interlock.
Brake Circuit
J2-1
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