Hydraulic Brake Control Valves

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Hydraulic Brake Control Valves

• Automotive Mechanics Technology
• Walla Walla Community College

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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

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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

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Metering Valve

• Purpose of Valve
• Location/construction of metering valve
• Operation of metering valve
• Bleeding system with a metering valve

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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

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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
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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

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Height/Load Sensing Valve

• Some systems use one valve per rear wheel.

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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

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Combination Valve
Pg 111C

• Combines 2 or 3 functions
• Metering valve
• Warning switch
• Proportioning valve

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3 Function Combination Valve
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Hydraulic System Bleeding
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Brake Bleeding and Bleeder Screws

• Must remove all air from system
• Five methods
• Manual bleeding
• Pressure bleeding
• Vacuum bleeding
• Gravity bleeding
• Surge bleeding

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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

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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

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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

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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

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Metering Valve prep

• Must hold open valve when pressure bleeding
• Several tools available

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Wheel Brake Bleeding Sequence
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Pressure Bleeding Set Up
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Brake Hydraulic System Service

• Manufacturers 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!

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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.

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Re-centering Single Piston Valve with centering
springs
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Re-centering single piston valve without
centering springs
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Re-centering Two Piston Valve with Centering
Springs
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Servicing Hydraulic System Valves
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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)

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Proportioning Valve
Pg 171L

• 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

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Height Sensing Proportioning Valve
Pg 174L

• 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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That's All !