Service Manual User Manual

Section 9
9-20
9.8.1 Brake Circuit
The brake system circuit (Fig. 9– 12) includes the park
brake and the service brake, along with the auxiliary
function manifold, main control valve and various hoses,
fittings and other components.
With the vehicle at rest (ignition keyswitch OFF, engine
OFF), the park brake is ON.
The vehicle is designed so that the service brake can be
used to stop the vehicle with the engine OFF.
The the park brake itself is part of the front axle ONLY.
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
The front axle (only) is equipped with a spring-applied,
pressure-released park brake. The park brake operates
via two solenoids at the auxiliary function manifold. With
the engine running and the park brake switch ON, one of
the solenoids is normally open and the other is normally
closed. This prevents 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,
oil flows from the main hydraulic pump, to the auxiliary
function valve, through the park brake solenoid valve, out
the park brake port PB, to the front axle 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 solenoid valve at the
auxiliary function manifold.
With the engine OFF and the park brake switch OFF,
spring pressure at the front axle engages the park brake.
b. Service Brake Circuit
A hydraulic line from auxiliary function manifold port PS
(Fig. 9– 12) provides flow to the service brake valve when
the engine is running. A reservoir line is installed on the
service brake valve at port T. There are also lines for the
front wheel brakes, rear wheel brakes, and a shuttle in
line on the service brake valve. Pressure can be checked
at auxiliary function manifold port PSG and at the pres-
sure tap fitting installed in the rear brake line.
The service brake valve itself is a severe-duty type valve
used in a variety of agricultural and industrial applica-
tions. The valve actually consists of two identical brake
valves mounted next to each other in the same casting.
Each valve can operate independently of the other.
The service brake valve operates much the same as a
pressure reducing valve in the power mode. As force is
applied to the brake pedal, the valve sends oil to the vehi-
cle brakes at pressure that is proportional to the force
that is applied to the pedal.
In the manual mode of operation (engine OFF), the brake
valve operates as a two-stage pump. The first stage
pumps a high volume of oil up to a low amount of pres-
sure. The second stage then pumps a low volume of oil
for higher braking pressure.
At rest (or “standby”), system pressure exists at the ser-
vice brake valve, ready for pedal application.
Fluid Flow When Brake Pedal is NOT Pressed
System pressure from the main pump flows to the
unloader valve, then to the auxiliary function manifold.
From there, the oil is routed out to the brake valve,
located above the brake pedal and beneath the dash.
With the pedal NOT depressed, the spool valves are
closed and no oil is allowed to flow to the axles.
Model 3606 • Origin 10/99