UNDERCARRIAGE
RT765E-2 SERVICE MANUAL
8-14
Published 9-04-2014, Control # 422-08
tandem brake valve with treadle pedal, the dual accumulator
charge valve, two hydraulic accumulators, the brake
assemblies, and all the associated hoses and tubing. The
operator depresses the pedal on the tandem brake valve,
located on the cab floor, and the valve modulates the brake
line pressure to the brake assemblies at each wheel. The full
powered brake system supplies a high brake system
pressure with relatively low reactive pedal forces, while
controlling the maximum brake line pressure. The service
brake dual accumulator charge valve regulates flow to the
h y d r a u l i c a c c u m u l a t o r s t o p r o v i d e f u l l y p o w e r e d
independently separate, primary (front) and secondary
(rear), service brake circuits. Hydraulic pressure is
c o n s ta n t l y m a i n ta i n e d i n t h e b r a k e c i r c u i ts b y t h e
accumulators and the charging valve. The brake assemblies
are disc type brakes.
Parking Brake
The parking brake is a hydraulically-controlled, disc-type
brake, located on the front axle. It is spring applied and
hydraulically released. The system consists of a two-position
switch, a three-way solenoid valve, actuator, brake
assembly, and all the associated hardware and tubing. The
selector switch, located on the steering column in the cab, is
used to activate the solenoid valve which controls the park
brake actuator, which applies and releases the park brake.
NOTE:
For Description and Maintenance of the tandem
brake valve with treadle pedal, the accumulators,
and the dual accumulator charge valve, refer to
Theory of Operation
Service Brakes
Braking begins when the operator depresses the brake
pedal in the cab. Mechanical linkage transfers the force
created by the lever action of the brake pedal to the hydraulic
brake valve which modulates the brake line pressure to the
brake assemblies at each wheel.
Hydraulic oil from hydraulic pump number 3 flows to a 10
gpm (37.85 l/min) priority flow control valve then through
swivel port 8 to the dual accumulator charge valve. The dual
accumulator charge valve charges the accumulators from
the open center circuit upon demand and within its present
operating charge rate and the high limit pressure setting.
However, when the open center circuit pressure reaches the
b r a k e r e l i e f s e t t i n g , w h i c h i s h i g h e r t h a n t h e h i g h
accumulator charge limit, then the accumulators will be
charged to the relief valve setting. The dual accumulator
charge valve regulates flow to the hydraulic accumulators to
provide fully powered independently separate, primary
(front) and secondary (rear), service brake circuits. Hydraulic
pressure is constantly maintained in the brake circuits by the
accumulators and the charging valve. The charged
accumulators supply pressurized fluid to the closed tandem
brake valve.
After the accumulators are fully charged, the high limit check
opens and all of the pump flow is directed to the excess flow
port and on to the cab air conditioning circuit.
Once the operator depresses the brake pedal, the tandem
brake valve modulates fluid out to the brakes to provide the
means of braking. The tandem brake valve will modulate the
pressure in the brake system by increasing or decreasing
pressure as required in proportion to the input force from the
operator via the brake pedal. The hydraulic force acts within
the brake assemblies to force the brake pads against the
brake discs, acting to slow wheel rotation. Fully powered
separate primary (front) and secondary (rear) braking circuits
are provided with independent accumulators. A low pressure
warning switch is used to sense the accumulator pressures
and warn the operator through visual brake warning indicator
light on the cab console in the event the pressure in the
accumulators drops to an unsafe operating level. In the
event of engine failure, the accumulators are pre-charged
with dry nitrogen gas and properly sized to provide power-off
stopping capacity for secondary braking.
Parking Brake
Hydraulic flow from the transmission charge pump is routed
to the parking brake control valve. When the Park Brake
Switch is in the ON position, the parking brake solenoid valve
shifts to route flow from the hydraulic parking brake actuator
back to the transmission sump. The actuator spring pulls on
the lever on the brake assembly, applying the parking brake.
Reference Only
