SkyTrak 6042 Service Manual - Manual (Page 282)

Hydraulic System
8.50 Model 6042 Legacy Origin 7/02
8.8.19 Brake Circuit Description
The brake system circuit (Fig. 8-4) includes the park
brake and the service brake, along with the secondary
function manifold, main control valve and various hoses,
fittings and other components.
When the engine is OFF and the ignition key 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, Drive Shafts, Wheels and
Tires” for further information.
a. Park Brake Circuit
Only the front axle is equipped with a spring-
applied, pressure-released park brake. The
park brake operates via two solenoids at the
secondary 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 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 unloader 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 solenoid 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.
b. Service Brake Circuit (Fig. 8-4)
A hydraulic line from secondary function manifold “PS”
port provides fluid flow to the service brake valve when
the engine is running. A line returns oil back to the reservoir
from “T” port on the service brake valve. There are also
two lines for the rear wheel brakes and a load sense line
on the service brake valve. Pressure can be checked at
the secondary function manifold “PSG” port and at the
pressure 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 commercial applications.
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.
Fluid Flow When Brake Pedal is NOT Pressed
System pressure from the pump flows to the unloader
valve and then to the secondary function manifold. From
there, the oil is routed out to the brake valve “P” port. The
brake valve is 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.
Fluid Flow When Brake Pedal is Pressed
System pressure from the pump flows to the unloader
valve, to the secondary function manifold and to the
brake valve. With the pedal depressed, spool valves
open and oil is allowed to flow through the brake valve
and out to the axles. Valve operation occurs in two
modes: the power mode and the manual mode.
In the power mode of operation (engine ON), the service
brake valve operates much the same as a pressure-
reducing valve. As force is applied to the brake pedal, the
valve sends oil to the vehicle 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
pressure. The second stage then pumps a low volume of
oil for higher braking pressure.
At rest or standby, system pressure exists at the service
brake valve, ready for pedal application.
The steps that follow describe service brake valve function
in both of those modes.
Power Mode
When the engine is ON, the brakes operate in the power
mode.
1. Displacement of the brake pedal is transmitted to the
service brake valve (Fig. 8-22, 1) at the plunger (2).
2. The series spring (Fig. 8-22, 3) transmits plunger
displacement to the piston (4). The series spring
also absorbs slight pedal pressure variations and
helps to provide smoother braking.
3. At rest or standby, system pressure exists at the
service brake valve (Fig. 8-22, 1), and the lines are
vented to the reservoir. The reservoir venting is
restricted as pedal pressure is applied, causing the
piston to contact the piston sealing ring (5). The
piston (4) is blocked when the fluid return metering
notches (6) on the spool are covered by the spool
bore.
MT3590