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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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2009Pearson
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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
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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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.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

©©2008
2009Pearson
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KEY TERMS:
brake fluid level sensor • brake light switch
combination valve

electronic brake proportioning • expander
height-sensing proportioning valve
metering valve
pressure-differential switch • proportioning valve
residual check valve • slope • split point
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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2009Pearson
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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. 

The ball and spring in the 
check valve permit all the 
brake fluid to return to the 
master cylinder until the 
designated pressure is 
reached.
Figure 70–1 Most residual check valves are
located under the tubing seals in the master
cylinder outlet ports.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Continued
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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 70–2.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 70–2 The momentary drop in pressure created when the brakes are released can draw
air into the hydraulic system.


Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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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. 

Figure 70–3 The use of cup expanders is the main reason why residual check valves are not
used in most braking systems today.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman

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PRESSURE-DIFFERENTIAL SWITCH
(BRAKE WARNING SWITCH)
A pressure­differential 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 70–4 A red brake warning lamp.


It lights the brake warning 
indicator lamp in the event
of a “difference in pressure.”
See Figure 70–5.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 70–5 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 lamp
remains on:
1. Apply light
pressure to
the brake pedal.
2. Momentarily
open the bleeder
valve on the side
that did not fail.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Those procedures should center the pressure­differential switch 

valve in those vehicles that are not equipped with self­centering 
springs. 

Figure 70–6 The pressure-differential switch piston is used to provide the electrical ground for
the red brake warning light circuit.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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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 float­type 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 70–7 and 70–8.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 70–7
A movable contact brake fluid level switch.

Figure 70–8
A magnetic brake fluid level switch.


NOTE: Brake systems use either a pressure­differential switch or a low
brake fluid switch to light the dash red “brake” lamp, but not both.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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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 pressure­differential 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
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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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 Pressure­Differential 
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 pressure­differential 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
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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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.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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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 70–9.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 70–9 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 70–10 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 70–11 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 70–12 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 spring­loaded 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 70–13.

Continued
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Figure 70–13 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 70–14.

Continued
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Figure 70–14 At the split point, the proportioning valve piston closes the fluid passage
through the valve.

Continued
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