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BRAKE VALVE
The brake pedal assembly combines the dynamic
retarding control with service brake control functions.
As the pedal is depressed, the first portion of pedal
travel actuates an internal potentiometer which pro-
vides an electrical signal to the propulsion system,
signalling a request by the operator for retarding. The
electrical signal is modulated by the operator as he
continues to depress the pedal, increasing the level of
retarding effort from minimum to maximum. Further
pedal depression after maximum retarding request is
achieved, results in service brake application from
minimum to maximum when the pedal is fully de-
pressed. A slight increase in pedal resistance can be
felt when service brake actuation occurs. Additionally,
indicator lights on the overhead display panel in the
cab will turn on as dynamic retarding is applied. A
second lamp will turn on when service brakes are
applied.
NOTE: The following information discusses the opera-
tion, test and repair of the hydraulic brake valve as-
sembly only. Refer to Section "E" for information
concerning the retarding control portion of the pedal.
The Brake Valve is a pressure modulating valve, actu-
ated mechanically (brake pedal) or hydraulically
through the automatic apply valve (8, Figure 3-1).
The Brake Valve controls the pressure delivered to the
front and rear service brake dual relay valves which
provide the apply pressure for the front wheel and rear
wheel disc brake assemblies. Apply pressure can be
modulated from zero to maximum braking effort by use
of the foot pedal.
Rebuild Criteria
If any one of the following conditions exist, the brake
valve should be removed and repaired:
• Excessive cam rock in pedal actuator.
• Any sign of external leakage.
• Internal leakage at the tank port must be less
than 100 cc/minute with the valve in the re-
leased position and system pressure supplied
to the "P1" and "P2" inlet ports.
• Tank port leakage must be less than 250
cc/minute with valve pilot or manual applied at
2750 psi (18,960 kPa) system pressure.
• Failure of the pedal to return to full release po-
sition.
J03019 1/99
BRAKE CIRCUIT COMPONENT SERVICE
Brake Circuit Component Service
FIGURE 3-1. BRAKE ACCUMULATOR BLEED DOWN
1. Rear Brake Accumulator
2. Charging Valve
3. Front Brake Accumulator
4. Charging Valve
5. Brake Manifold
6. Accumulator Bleed Down Valve (Front)
7. Accumulator Bleed Down Valve (Rear)
8. Automatic Apply Valve
• Valve holds pressure when in the neutral posi-
tion.
• Varying output pressure with pedal fully de-
pressed.
J3-1
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