Hydraulic Brake Control Valves

Automotive Mechanics Technology

Walla Walla Community College

Objectives

Identify purpose of metering valve Identify purpose of proportioning valve Identify purpose of pressure differential

valve Explain operation of height sensing

proportioning valve Identify Operation of combination valve

Introduction

Brake hydraulic systems need various type of control devices

Braking performance must be even and consistent

Combination disc/drum systems require different type of valves

Metering Valve

Purpose of Valve

Location/construction of metering valve

Operation of metering valve

Bleeding system with a metering valve

Metering Valve

Delays application of front discs slightly

Located between master cylinder and front disc brakes (pg 100C)

Operation Valve is held open by spring Closes at 3 – 30 psi Opens at 75 – 300 psi

Operation happens in microseconds

Proportioning Valve

Proportioning valve balances pressure on front disc, rear drum vehicles

Restricts fluid to the rear brakesLocated between MC and rear

brakes (pg 100C)Split point is the pressure when

P.V. closes Slope is percentage of rear

pressure to total pressureTwo proportioning valves

required on diagonal split system

Some 4 wheel disc brake cars have two Proportioning valves

Page 104 – 105C

Height Sensing Proportioning Valve

Used on trucks and some cars

Reduces pressure to rear brakes for light loads

Two types of valveslinkage type (pg 107C)Gravity ball (pg 108C)

Placement of valves

Height/Load Sensing Valve

Some systems use one valve per rear wheel.

Pressure Differential Valve

Hydraulically operated switch

Located between front and rear systems

Some systems use a low fluid level switch instead of a pressure Diff Switch

Combination Valve

Combines 2 or 3 functionsMetering valveWarning switchProportioning valve

Pg 111C

3 Function Combination Valve

Hydraulic System Bleeding

Brake Bleeding and Bleeder Screws

Must remove all air from system

Five methods Manual bleeding Pressure bleeding Vacuum bleeding Gravity bleeding Surge bleeding

Symptoms of Trapped Air

Low and Spongy when first applied Repeated pedal pumping will make pedal

become firm Can be similar to drum brakes in need of

adjustment Can perform air entrapment test to verify air

in system

Air Entrapment Test

Wear safety glasses Pump brake 10 to 20 times rapidly Hold pedal pressure Observe reservoirs Let off pedal If air in system, air will squirt fluid

Bleeding Procedures

Must follow bleeding sequence and procedures (pg 137L) Bench bleed master cylinder first Start at high points in system Then, continue from farthest point away from MC

and work back towards MC. Watch fluid level! Use jar and hose to observe fluid

Frozen Bleeder Screw

Use six sided wrench or socket

Always test bleeders first when doing brake work

Sometimes a hammer will break bleeder free

Sometimes use applied heat

Tapping on caliper will also free air bubbles

Metering Valve prep

Must hold open valve when pressure bleeding

Several tools available

Wheel Brake Bleeding Sequence

Pressure Bleeding Set Up

Brake Hydraulic System Service

Manufacturer’s RecommendationsFollow brake inspection scheduleFollow service procedures

Problems of FlushingSystem should be refilled with fresh fluid

Intervals are 15,000 to 60,000 milesBe careful of stirring sludge up in system!

Recentering a Pressure Differential Valve

After flushing or work on brake system valve becomes off center

Make sure other sources not turning on light

Must determine type of Pressure Differential

Valve

Valve may be separate or part of M.C.

Re-centering Single Piston Valve with centering springs

Re-centering single piston valve without centering springs

Re-centering Two Piston Valve with Centering Springs

Servicing Hydraulic System Valves

Metering Valves

Brake problems caused by bad metering valveCan allow front brakes to lock prematurely

Testing metering valveInspect boot for leakage

Metering valve pressure test (2 methods)1. Apply brakes and feel stem 2. Pressurize system over 40 psi and open front bleeder to see if system fluid flows (should not flow)

Proportioning Valve

Brake problems caused by bad proportioning valve

May cause rear drum lock up

Testing proportioning valve

Check for leaks

Check “split point” pressure operation (pg 171L)

Servicing of proportioning valve

Can only overhaul non integral valves

Pg 171L

Height Sensing Proportioning Valve

Brake problems caused by bad valveCan cause over braking or reduced brakingChassis modifications

Adjustment of valveFollow factory procedures

Removal and replacement of valveRoad test to make sure correct operation

Pg 174L