Lull 944E-42, Service Manual

Page: 397 / 844

8.147 Model 644E-42/944E-42 Rev. 6/04
Hydraulic System
8.8.50 Brake Circuit Description
The brake system circuit (see pages 8.68 thorough 8.72)
includes the park brake and the service brakes, along
with the main control valve, accumulator charge/
secondary function valve, accumulators and various
hoses, fittings and other components.
When the engine is OFF and the ignition switch is in the
OFF position, the park brake is ON. The park brake is part
of the front axle ONLY.
The vehicle is designed so that the service brakes can be
used to stop the vehicle with the engine OFF. The service
brakes are contained within both the front and rear axles.
Refer to Section 5, “Axles, Drive Shafts, Wheels, Tires
and Brakes,” for further information.
a. Park Brake Circuit (See page 8.68)
Only the front axle is equipped with a Spring-
Applied, Pressure-Released Park Brake. The
Park Brake operates via the Solenoid at the
Accumulator Charge/Secondary Function Valve.
Parking Brake ON (No Oil Pressure)
With the following conditions met:
• Ignition Switch OFF
OR
• Ignition Switch in the RUN position and Parking
Brake Switch ON
It prevents oil flow to the park brake in the axle allowing
the parking brake springs to engage the disks and plates
inside the parking brake. Oil pressure from the springs
flow back through the hose to the PB Port of the
Accumulator Charge/Secondary Function Valve, passing
through the Parking Brake Solenoid Valve and out the
“Drain” Port to the Hydraulic Reservoir.
Parking Brake OFF (Oil Pressure to Park Brake)
With the Engine Running and the Park Brake Switch
OFF, oil flows from the Primary section of the Hydraulic
Gear Pump, to the “P” Port of the Accumulator Charge/
Secondary Function Valve, through the Park Brake
Solenoid Valve, out the “PB” Port to the Park Brake
Section of the Front Axle. Oil pressure overcomes the
Spring Pressure within the Front Axle, and the Park
Brake is released. Oil is prevented from flowing back to
the reservoir because of the Check Valve in the
Accumulator Charge/Secondary Function Valve.
b. Service Brake Circuit (See page 8.70)
With the engine running, oil flows from the Hydraulic
Pump to the Accumulator Charge/Secondary Function
Valve, Port “P.” Two hydraulic lines from the Accumulator
Charge/Secondary Function Valve Ports, “ACR” and
“ACF,” charge the two Accumulators which provide oil
flow to the Service Brake Valve when the engine is
running.
Return lines from the “T” Port on the Service Brake Valve
and the “Drain” Port from the Accumulator Charge/
Secondary Function Valve return oil back to the Hydraulic
Reservoir.
From the Brake Valve, there are two independent lines
going to the Front Axle and Rear Axles, providing
pressure to the Service Brakes. Pressure can be checked
at the Brake Valve for each brake line.
This system utilizes two Accumulators to hold pressure in
the Service Brake System in the event of loss of engine
power. The Accumulators hold a sufficient charge to stop
the vehicle without engine power. The charge should be
sufficient for five pedal pushes. See “Checking
Accumulator Recharge Pressure and Nitrogen Pre-
Charge” on page 8.210.
c. Emergency Towing Park Brake Release Circuit
(See page 8.72)
With the engine OFF, ignition switch ON and the park
brake switch OFF, the park brake can be released
manually by attaching a Port-a-Power Hydraulic
Pressurizing Device to the Accumulator Charge/
Secondary Function Valve at the “PBG” port on the
Accumulator Charge/Secondary Function Valve. This
operation is used for emergency towing of the vehicle
only.
Use the Port-a-Power Hydraulic Pressurizing Device to
pump hydraulic pressure into the Park Brake System. DO
NOT exceed 650 psi (45 bar) when pressurizing the
system.
When releasing the park brake with the Port-a-Power
Hydraulic Pressurizing Device, pressure is applied to the
Park Brake Circuit through the “PBG” Port on the
Accumulator Charge/Secondary Function Valve out the
“PB” Port to the Park Brake in the Front Axle.
MT3590
CAUTION:
DO NOT exceed 650 psi
(45 bar) when pressurizing the system.
Applying too much pressure may damage the
brake seals.