SkyTrak 8042 Service Manual - Manual (Page 473)

Hydraulic System
8.58 Model 8042, 10042, 10054 Legacy Rev. 10/03
8.8.21 Brake Circuit Description
The brake system circuit, Section 8.8.1, “Brake Circuit
and Troubleshooting,” 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 (refer to page 8.18)
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 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 (refer to page 8.18)
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 main control
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 main control
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 the 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 (1) at the plunger (2).
2. The series spring (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 (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