Manual Drivetrains and Axles Fourth Edition

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start
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OBJECTIVES
After studying Chapter 70, the reader should be
able to

• Prepare for ASE Brakes (A5) certification test
  content area A (Hydraulic System Diagnosis and
  Repair).
• Describe the operation of a residual check valve.
• Explain how a proportioning valve works.

Continued
3
OBJECTIVES
After studying Chapter 70, the reader should be
able to

• Discuss the need and use of a metering valve.
• List testing procedures used to test hydraulic
  valves.
• Describe how the brake fluid level and brake
  light switches work.

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KEY TERMS

• brake fluid level sensor brake light
  switchcombination valveelectronic brake
  proportioning expanderheight-sensing
  proportioning valvemetering valvepressure-diff
  erential switch proportioning valveresidual
  check valve slope split point

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RESIDUAL CHECK VALVE

• A residual check valve has been used on some drum
  brake systems to keep a slight amount of pressure
  on the entire hydraulic system for drum brakes (5
  to 12 psi).

The check valve is located in the master cylinder
at the outlet for the drum brakes.
.
Figure 701 Most residual check valves are
located under the tubing seals in the master
cylinder outlet ports.
Continued
6

• This slight pressure prevents air leaks from
  entering into the hydraulic system in the event
  of a small hole or leak. With a low pressure
  kept on the hydraulic system, any small hole will
  cause fluid to leak out rather than permit air to
  enter the system.This slight pressure also
  keeps the wheel cylinder sealing cups tight
  against the inside wall of the wheel
  cylinder.See Figure 702.

Continued
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Figure 702 The momentary drop in pressure
created when the brakes are released can draw air
into the hydraulic system.
Continued
8
Figure 703 The use of cup expanders is the main
reason why residual check valves are not used in
most braking systems today.

• Residual check valves are often not used on late
  model vehicles equipped with front disc/rear drum
  brakes.The residual check valve has been
  eliminated by equipping the wheel cylinder
  internal spring with a sealing cup expander to
  prevent sealing cup lip collapse.

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PRESSURE-DIFFERENTIAL SWITCH(BRAKE WARNING
SWITCH)

• A pressure-differential switch is used on all
  vehicles built after 1967 with dual master
  cylinders to warn the driver of a loss of
  pressure in one of the two separate systems by
  lighting the dashboard red brake warning
  indicator lamp.

The brake lines from both front and rear sections
of the master cylinder are sent to this switch
Figure 704 A red brake warning lamp.
It lights the brake warning indicator lamp in the
eventof a difference in pressure.
See Figure 705.
Continued
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Figure 705 A leak in the hydraulic system
causes unequal pressures between the two
different brake circuits. This difference in
pressures causes the plunger inside the
pressure-differential switch to move, which
completes the electrical circuit for the red
brake warning lamp. (Courtesy of Ford Motor
Company)

• If the lampremains on

1. Apply lightpressure tothe brake pedal.
2. Momentarilyopen the bleedervalve on the
   sidethat did not fail.

Continued
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Figure 706 The pressure-differential switch
piston is used to provide the electrical ground
for the red brake warning light circuit.

• Those procedures should center the
  pressure-differential switch valve in those
  vehicles that are not equipped with
  self-centering springs.

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BRAKE FLUID LEVEL SENSOR SWITCH

• Many master cylinders, especially systems that
  are a diagonal split, usually use a brake fluid
  level sensor or switch in the master cylinder
  reservoir.This sensor will light the red
  brake warning lamp on the dash if low brake
  fluid level is detected. A float-type sensor or a
  magnetic reed switch are commonly used and
  provide a complete electrical circuit when the
  brake fluid level is low.After refilling the
  master cylinder reservoir to the correct level,
  the red brake warning lamp should go out.
• See Figures 707 and 708.

Continued
13
Figure 707A movable contact brake fluid level
switch.
Figure 708A magnetic brake fluid level switch.
NOTE Brake systems use either a
pressure-differential switch or a low brake fluid
switch to light the dash red brake lamp, but
not both.
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DIAGNOSING A RED BRAKE DASH WARNING LAMP

• Activation of the red brake dash warning lamp can
  be for any one of several reasons

1. Parking Brake On. The same dash warning lamp
is used to warn the driver that the parking brake
is on. 2. Low Brake Fluid. This lights the red
dash warning lamp on vehicles equipped with a
master cylinder reservoir brake fluid level
switch. 3. Unequal Brake Pressure. The
pressure-differential switch is used on most
vehicles with a front/rear brake split system to
warn the driver whenever there is low brake
pressure to either the front or rear brakes.
Continued
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• The likely cause of the red brake warning lamp
  is low brake fluid caused by a leaking brake
  line, wheel cylinder, or caliper. The first step
  is to determine the cause of the lamp being on,
  then to repair the problem.

Step 1 Check the Level of the Brake Fluid If
low, carefully inspect entire hydraulic brake
system for leaks repair as necessary.Step 2
Disconnect the Wire from the Pressure-Differential
Switch If the lamp is still on, the problem
is the parking brake lever switch, or the wire
going to the switch, shorted to ground.If the
red brake warning lamp is off after being
disconnected from the pressure-differential
switch, the problem is due to a hydraulic
failure (a low pressure in either front or rear
system creating a pressure difference of at least
150 psi).
Continued
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NOTE Many Japanese vehicles energize the relay
that turns off the red brake warning lamp from
the output terminal of the alternator. If a quick
inspection of the brake system seems to indicate
that everything is okay, check for correct
charging voltage before continuing a more
detailed brake system inspection.
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PROPORTIONING VALVE

• A proportioning valve improves brake balance
  during hard stops by limiting hydraulic pressure
  to the rear brakes.The valve is necessary
  because inertia creates weight shift toward the
  front of the vehicle during braking. The shift
  unloads the rear axle, which reduces traction
  between the tires and the road, and limits the
  amount of stopping power that can be
  delivered.Unless application pressure to the
  rear wheels is limited, the brakes will lock,
  making the vehicle unstable and likely to spin.
  The best overall braking performance is achieved
  when the front brakes lock just before the rear
  brakes.
• See Figure 709.

Continued
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Figure 709 Many proportioning valves are
mounted directly to the master cylinder in the
outlet to the rear brakes.
Continued
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• Vehicles with front disc and rear drum brakes
  require a proportioning valve for two reasons

1. Disc brakes require higher hydraulic pressure for
   a given stop than do drum brakes. In a disc/drum
   system, the front brakes always need more
   pressure than the rear brakes.
2. Once braking has begun, drum brakes require less
   pressure to maintain a fixed level of stopping
   power than they did to establish that level. In a
   disc/drum system, the rear brakes will always
   need less pressure than the front brakes.

A proportioning valve is used to compensate for
these differences because it is easier to reduce
pressure to the rear brakes than to increase
pressure to the front brakes.
Continued
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Figure 7010 Typical proportioner valve pressure
relationship. Note that, at low pressures, the
pressure is the same to the rear brakes as is
applied to the front brakes. After the split
point, only a percentage (called the slope) of
the master cylinder pressure is applied to the
rear brakes.

• The proportioning valve does not work at all
  times.

During light or moderate braking, there is no
rear wheel locking a problem.
Before proportioning will begin, hydraulic
pressure must reach a level called the split
point.
Continued
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Figure 7011 A Chrysler proportioning valve.
Note that slope and split point are stamped on
the housing.

• Above the split point,the proportioning valve
  allows only a portion of the pressure through to
  the rear brakes.

Figure 7012 These two proportioning valves are
found under the vehicle on this Dodge minivan.
Below the split point full system pressure is
supplied to the rear brakes.
Continued
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• Proportioning Valve Operation A simple
  proportioning valve consists of a spring-loaded
  piston that slides in a stepped bore.The piston
  is exposed to pressure on both sides. The smaller
  end of the piston is acted on by pressure from
  the master cylinder, while the larger end reacts
  to pressure in the rear brake circuit.The
  actual proportioning valve is located in the
  center of the piston and is opened or closed
  depending on the position of the piston in the
  stepped bore.
• See Figure 7013.

Continued
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Figure 7013 The proportioning valve piston can
travel within the range shown without reducing
pressure to the rear brakes.
Continued
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• When the brakes are first applied, hydraulic
  pressure passes through the proportioning valve
  to the rear brakes.Hydraulic pressure is the
  same on both sides of the piston, but because the
  side facing the rear brakes has more surface area
  than the side facing the master cylinder, greater
  force is developed and the piston moves to the
  left against the spring tension.At pressures
  below the split point, the proportioning valve is
  open, and pressure to both the front and rear
  brakes is the same. As the vehicle is braked
  harder, increased system pressure forces the
  piston so far to the left that the proportioning
  valve is closed.See Figure 7014.

Continued
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Figure 7014 At the split point, the
proportioning valve piston closes the fluid
passagethrough the valve.
Continued
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Always Inspect Both Front and Rear Brakes

• If a vehicle tends to lock up the rear brakes
  during a stop, many techs may try to repair the
  problem by replacing the proportioning valve or
  servicing the rear brakes. Proportioning valves
  are simple spring-loaded devices that are usually
  trouble free. If the rear brakes lock up during
  braking, carefully inspect the rear brakes
  looking for contaminated linings or other
  problems that can cause the rear brakes to grab.

Do not stop therealways inspect the front
brakes, too. If the front brakes are rusted or
corroded, they cannot operate efficiently and
greater force must be exerted by the driver to
stop the vehicle. Even if the proportioning valve
is functioning correctly, the higher brake pedal
pressure by the driver could easily cause the
rear brakes to lock up.
A locked wheel has less traction with the road
than a rotating wheel. As result, if the rear
wheels become locked, the rear of the vehicle
often comes around or fishtails, causing the
vehicle to skid. Careful inspection of the entire
braking system is required to be assured of a
safe vehicle.
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HEIGHT-SENSING PROPORTIONING VALVES

• Many vehicles use a proportioning valve that
  varies the amount of pressure that can be sent to
  the rear brakes depending on the height of the
  rear suspension. This type of valve is called a
  height-sensing proportioning valve.When the
  vehicle is loaded, the rear suspension is forced
  downward. The lever on the proportioning valve
  moves and allows a greater pressure to be sent to
  the rear brakes. This greater pressure allows
  the rear brakes to achieve more braking force,
  helping to slow a heavier vehicle. When heavily
  loaded in the rear, chances of rear wheel lockup
  are reduced.
• See Figures 7015 and 7016.

Continued
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Figure 7015 A height-sensing proportioning
valve provides the vehicle with variable brake
balance. The valve allows higher pressure to be
applied to the rear brakes when the vehicle is
heavily loaded and less pressure when the vehicle
is lightly loaded.
Continued
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Figure 7016 A stepped cam is used to alter the
split point of this height-sensing proportioning
valve.
CAUTION Some vehicle manufacturers warn that
service techs should never install replacement
air lift shock absorbers or springs that result
in a vehicle height difference than specified by
the manufacturer.
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PROPORTIONING VALVE ADJUSTMENT

• Height-sensing proportioning valves should be
  adjusted when replaced. The proper adjustment
  ensures that the proper pressure is applied to
  the rear brakes in relation to the loading of the
  vehicle.Procedures vary from one vehicle to
  another. Always consult the factory service
  information for the exact procedure. Some trucks
  require the use of special plastic gauges
  available from the dealer.

Continued
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PROPORTIONING VALVE DIAGNOSIS AND TESTING

• Whenever rear brakes tend to lock during braking,
  the proportioning valve should be checked for
  proper operation. If the proportioning valve is
  height sensing, verify the proper vehicle ride
  (trim) height and adjustment of the operating
  lever.Pressure gauges can also be used to check
  for proper operation. Install one gauge into the
  brake line from the master cylinder and the
  second gauge to the rear brake outlet of the
  proportioning valve. While an assistant depresses
  the brake pedal, observe the two gauges. Both
  gauges should register an increasing pressure as
  the brake pedal is depressed until the split
  point. See Figure 7017.

Continued
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Figure 7017 A proportioning valve pressure test
can be performed using two pressure gaugesone to
register the pressure from the master cylinder
and the other gauge to read the pressure being
applied to the rear brakes. This test has to be
repeated in order to read the pressure to each
rear wheel.
Continued
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• A defective proportioning valve usually allows
  rear brake pressure to increase too rapidly,
  causing the rear wheels to lock up during hard
  braking. When the rear brakes become locked, the
  traction with the road surface decreases and the
  vehicle often skids. After the split point, the
  gauge connected to the proportioning valve (rear
  brakes) should increase at a slower rate than the
  reading on the gauge connected to the master
  cylinder.If the pressures do not react as
  described, the proportioning valve should be
  replaced. The same procedure can be performed on
  a diagonal split-type system as used on most
  front-wheel-drive vehicles.

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ELECTRONIC BRAKE PROPORTIONING

• The Delphi DBC-7 eliminates the need for a
  conventional brake proportioning valve. A
  proportioning valve is usually necessary to
  reduce pressure to the rear brakes to keep them
  from locking up. This is because there is less
  weight over the rear wheels, and weight shifts
  forward when braking.Proportioning is needed
  most when a vehicle is lightly loaded or braking
  from a high speed. Most valves are calibrated to
  reduce pressure to the rear brakes by a fixed
  amount, which may increase the risk of rear-wheel
  lockup if the vehicle is loaded differently or is
  braking on a wet or slick surface.Dynamic rear
  proportioning is overcome by adjusting balance to
  match the vehicle to changing road and load
  conditions.

Continued
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• Electronic brake proportioning in the DBC-7
  system is accomplished by monitoring front- and
  rear-wheel speeds, and reducing pressure to the
  rear brakes as needed using the ABS solenoids
  when there is a difference in wheel deceleration
  rates. The pump may also run to clear the
  accumulators if a sufficient number of release
  cycles are required.The dynamic rear
  proportioning function is enabled at all times
  unless there is a failure of the EBCM or two
  wheel speed sensors on the same axle both fail
  simultaneously.As long as there is at least one
  functional speed sensor on the front and rear
  axles, the Electronic Brake Control Module (EBCM)
  can compare the relative speeds of the front and
  rear wheels.

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METERING VALVE (HOLD-OFF) OPERATION

• A metering valve is used on all front-disc,
  rear-drum-brake-equipped vehicles. The valve
  prevents the full operation of (holds off) the
  disc brakes until between 75 and 125 psi is sent
  to the rear drum brakes to overcome rear-brake
  return spring pressure.This allows front and
  rear brakes to apply at the same time for even
  stopping. Most metering valves also allow for
  pressure tothe front brakes to be gradually
  blended up to the metering valve pressure to
  prevent front brake locking under light pedal
  pressures on icy surfaces. A metering valve
  consists of a piston controlled by a strong
  spring and a valve stem controlled by a weak
  spring.See Figure 7018.

Continued
37
Figure 7018 A metering valve when the brakes
are not applied. Notice the brake fluid can flow
through the metering valve to compensate for
brake fluid expansion and contraction that occurs
with changes in temperature.
Continued
38

• When the brakes are not applied, the strong
  spring seats the piston and prevents fluid flow
  around it. At the same time, the weak spring
  holds the valve stem to the right and opens a
  passage through the center of the piston.When
  the brakes are applied and pressure in the front
  brake line reaches 3 to 30 psi (20 to 200 kPa),
  the tension of the weak spring is overcome and
  the metering valve stem moves to the left, which
  closes the passage through the piston and
  prevents fluid flow to the front brakes.See
  Figure 7019.

Continued
39
Figure 7019 A metering valve under light brake
pedal application.
Continued
40

• The small amount of pressure applied to the
  calipers before the metering valve closes is
  enough to take up any clearance, butnot enough
  to generate braking force.While the fluid flow
  to the front calipers is shut off, rear brake
  shoes move into contact with the drums, braking
  begins, and hydraulic pressure throughout the
  brake system increases.When the pressure at the
  metering valve reaches 75 to 300 psi, tension of
  the strong spring is overcome and the valve stem
  and piston move farther to the left.This opens
  a passage around the outside of the piston and
  allows fluid to flow through the valve to the
  front brake calipers.See Figure 7020.

Continued
41
Figure 7020 A metering valve during a normal
brake application.
Continued
42

No Valves Can Cause a Pull

• When diagnosing a pull to one side during
  braking, some technicians tend to blame the
  metering valve, proportional valve, the pressure
  differential switch, or the master cylinder
  itself.

Just remember that if a vehicle pulls during
braking that the problem has to be due to an
individual wheel brake or brake line. The master
cylinder and all the valves control front or rear
brakes together or diagonal brakes and cannot
cause a pull if not functioning correctly.
NOTE Neither the metering valve nor the
proportioning valve can cause a pull to one side
if defective. The metering valve controls both
front brakes, and the proportioning valve
controls both rear brakes. A defective master
cylinder cannot cause a pull either. Therefore,
if a vehicle pulls to one side during a stop,
look for problems in the individual wheel brakes,
hoses, or suspension.
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SYSTEMS WITHOUT METERING VALVES

• There are three reasons front-wheel-drive
  vehicles do not use metering valves

1. Front-wheel-drive vehicles usually have a more
   costly and complicated diagonally split dual
   braking system that requiresa separate metering
   valve for each hydraulic circuit.
2. Front-wheel-drive vehicles have a forward weight
   bias that requires front brakes to supply up to
   80 of the total braking power. A metering valve
   would create a delay.
3. Until all the clearance in the brake system is
   taken up, there will not be enough pressure in
   the brake hydraulic system for the front disc
   brakes to overcome the engine torque applied to
   the driven front wheels.

Continued
44

• Engine torque and a heavy front weight bias help
  prevent front wheel lockup from being a problem
  during light braking or when the brakes are first
  applied.Most rear-wheel-drive vehicles without
  metering valves are equipped with four-wheel disc
  brakes. Because the clearance between the pads
  and rotors is approximately the same at all four
  wheels, there is no need to delay front brake
  actuation.Some of these vehicles also have
  antilock brake systems that prevent the wheels
  from locking at any time.Other rear-wheel-drive
  vehicles without metering valves have a
  predominantly forward weight bias, like
  front-wheel-drive vehicles, and therefore benefit
  from having the front brakes applied sooner.

45
METERING VALVE DIAGNOSIS AND TESTING

• A defective metering valve can leak brake fluid
  and/or cause the front brakes to apply before the
  rear brakes. Most noticed on slippery surfaces
  such as on snow or ice or on rain-slick roads.

If the front brakes lock up during these
conditions, the front wheels cannot be steered.
Inspect the metering valve for these conditions

1. Look around the bottom of the metering valve for
   brake fluid leakage. (Ignore slight dampness.)
   Replace the metering valve assembly if it is
   leaking.
2. As the pressure builds to the front brakes, the
   metering valve stem should move. If it does not,
   replace the valve.

More accurate testing of the metering valve can
be accomplished using pressure gauges.
Continued
46

• Install two gauges, one in the pressure line from
  the master cylinder, the other in the outlet line
  leading to the front brakes.

When pressing the brake pedal, both gauges should
read the same until 3 to 30 psi (20 to 200 kPa)
when the metering valve shuts, delaying operation
of the front brakes. The master cylinder gauge
should show an increase in as the brake pedal is
pressed further.
Once 75 to 300 psi is reached, the gauge showing
pressure to the front brakes should match the
pressure from the master cylinder.If the
pressures do not match these ranges, the metering
valve assembly should be replaced.
NOTE Braking systems that are diagonal split,
such as those found on most front-wheel-drive
vehicles, do not use a metering valve. A
metering valve is only used on front/rear split
braking systems such as those found on most
rear-wheel-drive vehicles.
47

Push-In or Pull-Out Metering Valve?

• Whenever bleeding the air out of the hydraulic
  brake system, the metering valve should be
  bypassed. The metering valve stops the passage of
  brake fluid to the front wheels until pressure
  exceeds about 125 PSI (860 kPa). It is important
  not to push the brake pedal down with a great
  force so as to keep from dispersing any trapped
  air into small and hard-to-bleed bubbles.

To bypass the metering valve, the service
technician has to push or pull a small button
located on the metering valve. An easy way to
remember whether to push in or to pull out is to
inspect the button itself. If the button is
rubber coated, then you push in. If the button is
steel, then pull out.
Special tools allow the metering valve to be held
in the bypass position. Failure to remove the
tool after bleeding the brakes can result in
premature application of the front brakes before
the rear drum brakes have enough pressure to
operate.
48
COMBINATION VALVE

• Most manufacturers combine the function of a
  proportioning valve with one or more other valves
  into one unit called a combination valve.

On a rear-wheel-drive vehicle, a typical
combination valve consists of these components,
all in one replaceable unit

• Metering valve
• Proportioning valve
• Pressure-differential switch

Figure 702Typical two-function combination
valves.
Continued
49
Figure 7022 Combination valve containing
metering, pressure-differential (warning switch),
and proportioning valves all in one unit.

• Some valves have two functions and contain
  pressure-differential and metering valve others
  combine the pressure-differential with the
  proportioning valve.

This style is often calleda pistol grip design
because the valve section resembles the grip
section of a hand gun.
Continued
50
BRAKE LIGHT SWITCH

• The brake light switch turns on brake lights at
  the back of the vehicle when brakes are applied.
  A properly adjusted light switch will activate
  brake lights as soon as the pedal is applied and
  before braking action actually begins at the
  wheels.

Mechanical switches operating directly off the
brake pedal armare most often used.
Brake light switches are normally open when
brakes apply, the switch closes, completing the
light circuit.
Figure 7023 Typical brake light switches.
CAUTION Check service information for
procedures to follow when replacing and/or
adjusting a brake switch to ensure proper
operation.
51
SUMMARY

1. Residual check valves are used in older vehicles
   to keep a slight amount of pressure on the system
   to help prevent air from entering the system when
   the brake pedal is released.
2. A pressure-differential switch is used to turn on
   the red brake warning lamp in the event of a
   hydraulic pressure failure.
3. Brake fluid level sensors are used in many
   vehicles to warn the driver that the brake fluid
   level is low.
4. Proportioning valves are used to limit the
   maximum fluid pressure sent to the rear wheel
   brakes during heavy braking to help prevent rear
   wheel lockup.

Continued
52
SUMMARY
(cont.)

1. Metering valves are used on some vehicles to keep
   the front disc brakes from locking up on slippery
   surfaces.
2. Combination valves include two or more hydraulic
   valves in one assembly.

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end