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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 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