Operation and maintenance manual

Section 250-0000
Braking System - Braking System Schematic
SM 2155 Rev 1 10-03
2
Accumulator pressure is monitored by pressure
switches (4) in the brake lines.
Treadle Valve (6)
Refer to Section 250-0070, TREADLE VALVE.
The treadle valve controls the level of hydraulic fluid
pressure applied to front and rear brakes and the
maximum pressure available to these circuits. It is
operated by a foot pedal in the operators cab and, with
the engine running, is automatically applied by the
solenoid on the brake manifold valve.
The first 7
o
of pedal travel will activate the transmission
retarder. Further depression of pedal will apply service
brakes.
Stop Light Pressure Switch (7)
A pressure switch located at a tee in the 'B2' line
energizes (illuminates) the vehicle's stop lights on
application of the treadle valve.
Directional Control Valve (8)
Refer to Section 250-0090, DIRECTIONAL CONTROL
VALVE.
The directional control valve is mounted off the cradle
assembly at the rear of the cab alongside brake
manifold valve (2). It is located in the 'Px' hydraulic
circuit between brake manifold valve (2) and treadle
valve (6). The 'Px' circuit hydraulically actuates treadle
valve (6) when the park/emergency control switch is
activated. It also controls automatic service brake,
applied pressure bleed down, with engine shutdown.
Brake Accumulator (9)
The small brake accumulator, located between the
brake manifold valve (2) and the pilot supply valve, is
pre-charged to 30 bar (435 lbf/in²).
Priority Unloader Valve (10)
Refer to Section 250-0075, PRIORITY UNLOADER
VALVE.
Mounted off the cradle assembly at the rear of the cab,
the priority unloader valve distributes hydraulic oil flow
from the front section of the triple pump to the brake
manifold valve as it's first priority. Once the maximum
pressure is achieved in the brake circuit, the valve then
switches to supplement flow to the engine cooling fan
motor.
Note: Never drive a pump in the wrong direction of
rotation, as pump seizure may result.
Brake Manifold Valve (2)
Refer to Section 250-0050, BRAKE MANIFOLD
VALVE.
The brake manifold valve is mounted off the cradle
assembly at the rear of the cab alongside the
directional control valve. It distributes hydraulic oil flow
from the priority unloader valve, to the treadle valve,
front and rear brake circuit accumulators and parking
brake at the rear wheels. It also provides appropriate
returns to tank for the hydraulic oil.
Parking Brake Pressure Switch (3)
Located in the brake manifold valve body it senses
pressure in the parking brake (Px) line. The pressure
switch closes at a pressure of 4.9 bar (71 lbf/in²) and
sends a signal to illuminate the parking brake indicator
light when the parking brake is applied.
Brake Circuit Pressure Switches (4)
Front brake circuit pressure switch is located in a tee
piece below the front brake accumulator. The rear
brake circuit pressure switch is also located in a tee
piece below the rear brake accumulator. The pressure
switches sense pressure in the front and rear brake
circuits and send a signal to warning lights on the dash
(a buzzer also sounds) when the pressure drops below
115 bar (1 668 lbf/in²).
Brake Accumulators (5)
Refer to Section 250-0060, ACCUMULATOR.
There are two brake accumulators mounted on the
cradle assembly at the rear of the cab; one for the
front brake system and the other for the rear. The
accumulator is of the piston type and is precharged
with nitrogen to 55 bar (800 lbf/in²). It consists of a
charging valve assembly, cylinder assembly and
piston. The charging valve is equipped with a locking
feature which, when opened, will allow precharge to be
checked or accumulator charged.
The piston acts as a separator dividing the cylinder
assembly into two sections. The section nearest the
charging valve contains the nitrogen precharge.
Hydraulic oil from the priority unloader valve flows
through accumulator check valves in the brake
manifold valve and into the other section of the
accumulators.