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 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 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 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 • 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
Pg 111C Combination Valve Combines 2 or 3 functions Metering valve Warning switch Proportioning valve
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
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 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 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)
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
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