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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

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hydraulic brake control valves

Hydraulic Brake Control Valves

Automotive Mechanics Technology

Walla Walla Community College

objectives
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
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
Metering Valve

Purpose of Valve

Location/construction of metering valve

Operation of metering valve

Bleeding system with a metering valve

metering valve5
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

Proportioning valve balances pressure on front disc, rear drum vehicles

Restricts fluid to the rear brakes

Located between MC and rear brakes (pg 100C)

Split point is the pressure when P.V. closes

Slope is percentage of rear pressure to total pressure

Two proportioning valves required on diagonal split system

Some 4 wheel disc brake cars have two Proportioning valves

Page 104 – 105C

height sensing proportioning valve
Height Sensing Proportioning Valve

Used on trucks and some cars

Reduces pressure to rear brakes for light loads

Two types of valves

linkage type (pg 107C)

Gravity ball (pg 108C)

Placement of valves

height load sensing valve
Height/Load Sensing Valve
  • Some systems use one valve per rear wheel.
pressure differential valve
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

Pg 111C

Combination Valve

Combines 2 or 3 functions

Metering valve

Warning switch

Proportioning valve

brake bleeding and bleeder screws
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
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
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
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
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
Metering Valve prep
  • Must hold open valve when pressure bleeding
  • Several tools available
brake hydraulic system service
Brake Hydraulic System Service

Manufacturer’s Recommendations

Follow brake inspection schedule

Follow service procedures

Problems of Flushing

System should be refilled with fresh fluid

Intervals are 15,000 to 60,000 miles

Be careful of stirring sludge up in system!

recentering a pressure differential valve
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.

metering valves
Metering Valves

Brake problems caused by bad metering valve

Can allow front brakes to lock prematurely

Testing metering valve

Inspect 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 valve28

Pg 171L

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

height sensing proportioning valve29

Pg 174L

Height Sensing Proportioning Valve

Brake problems caused by bad valve

Can cause over braking or reduced braking

Chassis modifications

Adjustment of valve

Follow factory procedures

Removal and replacement of valve

Road test to make sure correct operation

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