Brake Circuit Description - JLG sky trak mmv Service Manual

Hydraulic System
9.8.24

Brake Circuit Description

The brake system circuit (see page 9.14 thru page 9.18)
includes the park brake and the service brake, along with
the secondary function manifold, main control valve, ac-
cumulator charge valve, accumulators and various hos-
es, 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 brake 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, Wheels and Tires for further
information.
a. Park Brake Circuit (See page 9.14)
Only the front axle is equipped with a spring-ap-
plied, pressure-released park brake. The park
brake operates via two solenoids at the second-
MT3590
ary function manifold. With the engine running
and the park brake switch ON, one of the solenoids is nor-
mally open and the other is normally closed. This pre-
vents fluid flow in one direction but allows it in another.
Spring pressure within the front axle engages the park
brake. Oil is prevented from flowing to the front axle and
releasing the park brake.
With the engine running and the park brake switch OFF,
fluid flows from the hydraulic pump, to the accumulator
charge valve, to the secondary function valve, through
the park brake solenoid valve, out the park brake "PB"
port and 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 closed sole-
noid valve at the secondary function manifold.
With the engine OFF and the park brake switch OFF,
spring pressure at the front axle engages the park brake.
9.76
b. Service Brake Circuit (See page 9.16)
With the engine, running fluid flows from the hydraulic
pump to the accumulator charge valve. Two hydraulic
lines from the accumulator charge valve "ACR" and
"ACF" charge the two accumulators which provide fluid
flow to the service brake valve when the engine is run-
ning. A line returns oil back to the reservoir from the "T"
port on the service brake valve. From the brake valve
there are lines going to the front axle and rear axles, pro-
viding 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.
c. Emergency Towing Park Brake Release Circuit
(See page 9.18)
With the engine OFF, ignition switch ON and the park
brake switch OFF, the park brake can be released man-
ually using the park brake release hand pump. This pump
is used for emergency towing of the vehicle only.
Note: With the ignition switch in the ON position, the low
brake pressure buzzer will sound continuously until the
towing operation is complete and ignition switch turned
to the OFF position.
CAUTION:
(45 bar) when pressurizing the system. Apply-
ing to much pressure may damage the brake
seals
Use the park brake release pump to pump hydraulic pres-
sure into the park brake system. Watch the pressure
gauge next to the hand pump. DO NOT exceed 650 psi
(45 bar) when pressurizing the system.
When releasing the park brake with the park brake re-
lease hand pump, oil is sucked out of the hydraulic oil res-
ervoir through the return line and is pressurized into the
park brake lines by the hand pump. The hand pump has
internal check valves to prevent oil from flowing back to
the tank when using the hand pump to release the brakes
or when the system is under normal operating pressure.
DO NOT exceed 650 psi
Model MMV Rev. 12/04