Skytrak 6042 Service Manual User Manual download pdf (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

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Skytrak 6042 Service Manual User Manual Page 282

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Skytrak 6042 Service Manual User Manual Page 282

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