chap80 pps Automotive technology at University of Cambridge

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

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OBJECTIVES:
After studying Chapter 80, the reader should
be able to:
•

Prepare for ASE Brakes (A5) certification test content area “D” (Power
Assist Units Diagnosis and Repair).

•
•
•

•

List the parts of a vacuum brake booster.
Describe how a vacuum brake booster operates.
Explain how to test a vacuum brake booster.
Describe how a hydraulic or electrohydraulic brake booster operates.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
atmospheric pressure • brake assist system (BAS)
dual-diaphragm vacuum booster
inches of mercury (in. Hg)

millimeters of mercury (mm Hg)
power chamber • pressure differential
supplemental brake assist (SBA)
tandem-diaphragm vacuum booster • vacuum

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THE NEED FOR POWER BRAKE ASSIST
To double the stopping power of a disc brake, the driver must
double the force on the brake pedal. This is the reason that most
vehicles equipped with disc brakes are power assisted. The most
commonly used power-assisted units are vacuum operated.
When a power booster is fitted, brake pedal ratio is decreased and

the master cylinder bore size is increased. The effect of these
changes is to reduce pedal effort, greatly increasing pedal reserve.
Power boosters do not alter the hydraulic system and allow braking
even if the booster fails or its power supply is cut off. All boosters
have a power reserve that provides assist for at least one hard stop,
and sometimes several light brake applications, even after power is
lost. See Figure 80–1.
Continued
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Figure 80–1 Typical vacuum brake booster assembly. The vacuum hose attaches to the intake
manifold of the engine. The brake pedal travel sensor is an input sensor for the antilock braking
system.

Continued
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Because power brake systems are designed with the added force of
the booster taken into account, the amount of brake pedal pressure
required to slow or stop a vehicle is much higher than in a
nonboosted system once the reserve is used up.
Figure 80–2 A wide brake pedal allows two
foot braking if power assist is lost.

For this reason, some vehicles
with power brakes have a brake

pedal that is wide enough to
allow two-foot braking should
the booster fail.

Continued
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PRINCIPLES OF VACUUM
Most vacuum-powered brake boosters get their vacuum supply
from the engine intake manifold. An engine is essentially a big
air pump because the pistons move up and down in the
cylinders to pump in air and fuel, and pump out exhaust.
As a piston moves down on intake with the intake valve open, it

creates a larger area inside the cylinder for air to fill
Although it may seem as though the low pressure is pulling air
into the engine, it is really the higher pressure outside that forces
air in. The difference in pressure between two areas is called a
pressure differential.
Continued
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Gasoline-powered internalcombustion engines normally
operate with a low-pressure area,
or partial vacuum, in the intake
manifold.

Figure 80–3 Atmospheric pressure varies with
altitude.

The term vacuum is used to
refer to any pressure lower than
atmospheric pressure.
Atmospheric pressure varies
with altitude, but is about 14.7
pounds per square inch (psi) at
sea level.

Continued
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Measuring Vacuum Vacuum is measured in inches of mercury
(in. Hg) or in millimeters of mercury (mm Hg), a figure that
indicates how far a column of mercury will rise when a vacuum
is applied at one end, and atmospheric pressure at the other.
Vacuum is a measurement of the pressure differential between the
lower pressure inside the tube, and the higher pressure outside it.
Manifold vacuum varies with throttle position. The lowest
vacuum (highest pressure) occurs when the throttle is wide
open with the engine under load.
The highest manifold vacuum (lowest pressure) may be as much
as 24 in. Hg (610 mm Hg). Vacuum at idle typically falls between
15 and 20 in. Hg (381 and 508 mm Hg). Most vacuum brake
boosters operate with vacuum levels in this range.
Continued
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Booster Vacuum Supply Vacuum boosters get their vacuum
supply from the engine intake manifold. Diesel engines, however,
run unthrottled (engine speed is controlled strictly by the amount of
fuel injected) and have little or no intake manifold vacuum.

If a vehicle with a diesel engine is
equipped with a vacuum-powered
brake booster, it must also be fitted
with an auxiliary vacuum pump.
Figure 80–4 A belt-driven auxiliary vacuum
pump.

Some small gasoline-powered
and diesel engines use a beltdriven add-on pump.
Continued
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An electrically powered vacuum pump is turned on and off by a
pressure switch on the booster. This means they operate only when
needed, and thus reduce power drain on the engine.

Figure 80–5 An electrically powered vacuum pump.

Continued
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VACUUM BOOSTER THEORY
Vacuum boosters use pressure differential to increase brake
application force. The typical booster has a power chamber
separated into two smaller chambers by a flexible diaphragm.
In an attempt to equalize pressure in the two chambers, the higher
pressure exerts force that moves the diaphragm toward the lower
pressure. Rods attached to the diaphragm transmit this force, plus
force the driver exerts on the brake pedal, to the master cylinder.
The amount of force created in this manner is proportional to the
difference in pressure between the two sides. In other words, the
greater the pressure differential, the greater the force.
Continued
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To calculate the force, the pressure differential is multiplied by the diaphragm surface
area. If a power booster diaphragm has atmospheric pressure (14.7 psi) on
one side, and a typical intake
manifold vacuum of 20 in. Hg
(10 psi of absolute pressure),
the pressure differential acting
on the diaphragm would be:

14.7 psi - 10 psi = 4.7 psi
If we multiply this times the area
of the diaphragm, the result is:
4.7 psi x 50 sq. in. = 235 pounds
of force
Figure 80–6 Vacuum brake boosters operate
on the principle of pressure differential.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Check the Vacuum, Then the Brakes - Part 1
A customer complained of very rough idle and an occasional pulsating brake
pedal. The customer was certain the engine required serious work since there were
over 100,000 miles on the vehicle. During the troubleshooting procedure, a spray
cleaner was used to find any vacuum leaks. A large hole was found melted
through a large vacuum hose next to the vacuum hose feeding the vacuumoperated power brake booster.
After repairing the vacuum leak, the vehicle was test driven again to help
diagnose the cause of the pulsating brake pedal. The engine idled very
smoothly after the leak was repaired and brake pulsation was also cured.
The vacuum leak resulted in lower-than-normal vacuum being applied to
the vacuum booster. During braking, when engine vacuum is normally
higher (deceleration), the vacuum booster would assist, then not assist
when the vacuum was lost. This on-and-off supply of vacuum to the
vacuum booster was noticed by the driver as a brake pulsation. Always
check vacuum at the booster whenever diagnosing any brake problems.

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

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Check the Vacuum, Then the Brakes - Part 2
Most manufacturers specify a maximum of 15 in. Hg of vacuum at the booster. It
should be able to provide two or three stops even with no vacuum. The booster
should also be checked to see if it can hold a vacuum after several hours. A
booster, for example, should be able to provide a power assist after sitting all night
without starting the engine.

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CHARCOAL FILTER
The vacuum hose leading from the engine to the power booster
should run downward without any low places.
If a dip or sag occurs in the hose, condensed fuel vapors and/or
moisture accumulate that block or restrict vacuum to the booster.
Many manufacturers use a small charcoal filter in the vacuum line
between the engine and booster. The filter attracts and holds fuel
vapors and keeps fumes from entering the vacuum booster.
Without this filter, gasoline fumes can enter the vacuum booster,
where it can deteriorate the rubber diaphragm and other rubber
components of the booster. See Figure 80–7.
Continued
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Figure 80–7 The charcoal filter traps gasoline vapors that are present in the intake manifold
and prevents them from getting into the vacuum chamber of the booster.

Continued
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VACUUM CHECK VALVE
All vacuum boosters use a one-way vacuum check valve. This
valve allows air to flow in only one direction—from the booster
toward the engine.
This valve prevents loss of vacuum when the engine stops.
Without this check valve, the vacuum stored in the vacuum
booster would simply be lost through the hose and intake
manifold of the engine.
See Figures 80–8 and 80–9.

Continued
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Figure 80–8 (a) Many vacuum-brake-booster check valves are located where the vacuum hose
from the engine (vacuum source) attaches to the vacuum booster. (b) This one-way valve
prevents the loss of vacuum when the engine is off. The diaphragm inside allows air to flow in
one direction only.

(a)

Continued

(b)

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 80–9 Not all check valves are located at the vacuum line to the booster housing
connection. This vehicle uses an inline check valve located between the intake manifold of the
engine and the vacuum brake booster.

Continued
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CAUTION: Sometimes an engine backfire can destroy or blow the

vacuum check valve out of the booster housing. If this occurs, all power
assist will be lost and a much greater-than-normal force must be exerted on
the brake pedal to stop the vehicle. Be sure to repair the cause of the
backfire before replacing the damaged or missing check valve. Normal
causes of backfire include an excessively lean air–fuel ratio or incorrect
firing order or ignition timing.

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VACUUM BRAKE BOOSTER OPERATION
A vacuum power-brake booster contains a rubber diaphragm(s)
connected to the brake pedal at one end and to the master cylinder
at the other end. When the brakes are off or released, there is equal

vacuum on both sides of the diaphragm.
The vacuum power unit contains the power-piston assembly,
which houses the control valve and reaction mechanism, and the
power-piston return spring.
An air filter, air silencer, and filter retainer are assembled around
the valve operating rod, inside the hub of the power piston. The
pushrod that operates the air valve projects out of the end.
See Figure 80–10.
Continued
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Figure 80–10 Cross-sectional view of a typical vacuum brake booster assembly.

Continued
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Released-Position Operation At the released position (brake pedal
up), the air valve is seated on the floating control valve, which shuts
off the air. The floating control valve is held away from the valve
seat in the power-piston insert.
Vacuum is present in the space on both sides of the power piston.
There is a vacuum on both sides of the power piston, and it is held
against the rear of the housing by the power-piston return spring.
At rest, the hydraulic reaction plate is held against the reaction
retainer. The air-valve spring holds the reaction lever against the
hydraulic reaction plate and holds the air valve against its stop in the

tube of the power piston.
The floating control-valve assembly is held against the air-valve
seat by the control-valve spring. See Figure 80–11.
Continued
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Figure 80–11 In the release position (brake pedal up), the vacuum is directed to both sides of
the diaphragm.

Continued
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