chap72 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 72, the reader should
be able to:

•
•
•

Prepare for ASE Brakes (A5) certification test
content area “A” (Hydraulic System
Diagnosis and Repair).

Explain how to bench bleed a master cylinder.
Describe the proper brake bleeding sequence.

Continued
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OBJECTIVES:
After studying Chapter 72, the reader should
be able to:

•
•
•

Describe the single stroke manual brake
bleeding procedure.
Discuss how to gravity bleed the hydraulic
brake system.
List the steps needed to perform a pressure
bleed procedure.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
bleeder valve • brake bleeding
gravity bleeding

power bleeding • pressure bleeding
reverse fluid injection
single stroke bleeding method • surge bleeding
vacuum bleeding

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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BRAKE BLEEDING
Brake bleeding is removing any trapped air from the hydraulic 
system. Air can get into the hydraulic system whenever any 
hydraulic brake line or unit is opened. A common source of air in 
the brake system of this type can occur through very small holes in 
rubber flexible brake lines.

Another source is absorption of moisture by brake fluid. When 
moisture is absorbed, boiling point of the brake fluid is reduced. 
During severe braking, the heat generated can cause the brake 
fluid to boil and create air bubbles in the hydraulic brake system.
Air eventually travels to the highest part of the brake system, if not 
restricted by pressure control valves. Air in the system results in a 
spongy brake pedal.
Continued
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BLEEDING THE MASTER CYLINDER
Whenever the master cylinder is replaced or the hydraulicsystem 
has been left opened for several hours, the air may have to be bled 

from the master cylinder. Bleed the master cylinder “on the 
bench” before installing it on the vehicle.

Figure 72–1 Always clamp a master
cylinder in a vise by the mounting
flange to prevent distortion of the
cylinder bore.

Continued
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If bleeding the master cylinder after working on the hydraulic 
system, follow these steps:

Step #1  Fill the master cylinder with clean brake fluid from a 
sealed container up to the recommended “full” level.
Step #2  Have an assistant slowly depress the brake pedal as you 
“crack open” the master cylinder bleed screw starting with the 
section closest to the bulkhead. It is very important that the 
primary section of the master cylinder be bled before attempting 
to bleed the air out of the secondary section of the master cylinder. 
Before the brake pedal reaches the floor, close the bleeder valve.
Step #3  Repeat several times until a solid flow of brake fluid is 
observed leaving the bleeder valve. If the master cylinder has no 
bleeder valves, outlet tube nuts can be loosened instead.
Continued
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NOTE: A proper manual bleeding of the hydraulic system requires that

accurate communications occur between the person depressing the brake
pedal and the person opening and closing the bleeder valve(s). The
bleeder valve (also called a bleed valve) should be open only when the
brake pedal is being depressed. The valve must be closed when the brake
pedal is released to prevent air from being drawn into the system.

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BRAKE BLEEDER VALVE LOOSENING
METHODS

Attempting to loosen a bleeder valve often results in breaking 
(shearing off) the bleeder valve. Several procedures can be tried 
that help prevent the possibility of breaking a bleeder valve. 
Bleeder valves are tapered and become wedged in the caliper on 
the wheel cylinder housing. 
Figure 72–2 Typical bleeder valve from a
disc brake caliper. The arrows point to
the taper section that does the actual
sealing. It is this taper that requires a
shock to loosen. If the bleeder is simply
turned with a wrench, the bleeder usually
breaks off because the tapered part at the
bottom remains adhered to the caliper or
wheel cylinder. Once loosened, brake
fluid flows around the taper and out
through the hole in the side of the bleeder
valve.
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Figure 72–3 Typical bleeder locations. Note that the combination valve and master cylinder
shown do not have bleeder valves; therefore, bleeding is accomplished by loosening the brake
line at the outlet parts. (Courtesy of Allied Signal Automotive Aftermarket)

Continued
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Air Impact Method  Use a 6­point socket for the bleeder valve and 
necessary adapters to fit an air impact wrench to the socket.
Apply some penetrating oil to the bleeder valve and allow it to flow 
around the threads.
Turn the pressure down on the impact wrench to limit the force. 
The hammering effect of the impact wrench loosens the bleeder valve 
without breaking it off.

Continued
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Hit and Tap Method 

Step #1  Tap on the end of the bleeder valve with a steel hammer. 
This shock often “breaks the taper” at the base of the bleeder 
valve. Shock also breaks loose rust or corrosion on the threads.
Step #2  Using a 6­point wrench or socket, tap the bleeder valve 
in the clockwise direction (tighten).
Step #3  Using the same 6­point socket or wrench, tap the bleeder 
valve counterclockwise to loosen and remove the bleeder valve.
Step #4  If valve is still stuck (frozen), repeat Step 1 thru Step 3.
NOTE: The shock of the tap on the wrench breaks loose the valve. 

Simply pulling on the wrench often results in breaking off the bleeder.
Continued
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Air Punch Method Use an air punch near the bleeder valve while 
attempting to loosen the bleeder valve at the same time.
The punch creates a shock motion that often loosens the taper and 
threads of the bleeder valve from the caliper or wheel cylinder. 
It is often helpful to first attempt to turn the bleeder valve in the 
clockwise (tightening), then turn in the counterclockwise direction 
to loosen and remove the bleeder valve.

Figure 72–4 Using an air punch next to the
bleeder valve to help “break the taper” on
the bleeder valve.

Continued
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Heat and Tap Method  Heat the area around the bleeder valve with 
a torch. The heat expands the size of the hole and usually allows the 
bleeder to be loosened and removed.
CAUTION: The heat from a torch will damage the rubber seals inside 

the caliper or wheel cylinder. Using heat to free a stuck bleeder valve will 
require that all internal rubber parts be replaced.

Continued
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Wax Method 
Step #1  Heat the bleeder valve itself with a torch. The heat causes 
the valve itself to expand.
Step #2  Remove heat from the bleeder valve. As the valve is 
cooling, touch paraffin wax or candle wax to the hot valve. The wax 
will melt and run down around the threads of the valve.
Step #3  Allow the bleeder valve to cool until it can be safely 
touched with your hand. This assures that the temperature is low 
enough for the wax to return to a solid and provide the lubricating 
properties necessary for the easy removal of the bleeder valve.
Again, turn the bleeder valve clockwise before turning the valve 
counterclockwise to remove.
Continued
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Do It Right—Replace the Brake Fluid
Often, used brake fluid looks like black coffee or coffee with cream. Both
conditions indicate contaminated or moisture-laden brake fluid that should
be replaced. The following steps will assure a complete brake fluid change:
Step #1 Remove the old brake fluid from the master cylinder using
a suction bulb. (Dispose of this old brake fluid properly.)
Step #2 Fill the master cylinder with new clean brake fluid from a
sealed container.
Step #3 Bleed each wheel brake until the brake fluid is clean.
This fluid replacement will fully restore the brake hydraulic system to asnew condition and protect the system from rust and corrosion. Replacing
only friction pads and/or linings is not a complete, thorough brake system
service. Customers should be aware of the importance of this procedure.

CAUTION: Do not allow the master cylinder to run out of brake fluid. 
Recheck and refill as necessary during the bleeding process.
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BLEEDING SEQUENCE
After bleeding the master cylinder, the combination valve should 
be bled if equipped.
Follow the same procedure as when bleeding the master cylinder, 
being careful not to allow the master cylinder to run dry.
NOTE: The master cylinder is located in the highest section of the 

hydraulic braking system. Some master cylinders are equipped with 
bleeder valves. All master cylinders can be bled using the same procedure 
as that used for bleeding calipers and wheel cylinders. If the master 
cylinder is not equipped with bleeder valves, it can be bled by loosening 
the brake line fittings at the master cylinder.

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Check the level in the master cylinder frequently and keep it filled 
with clean brake fluid throughout the brake bleeding procedure.
For most rear­wheel­drive vehicles equipped with a front/rear split 
system, start the bleeding with the wheel farthest from the master 
cylinder and work toward the closest. 
Figure 72–5 Most vehicle manufacturers
recommend starting the brake bleeding
process at the rear wheel farthest from
the master cylinder.

For most vehicles, this 
sequence is:
1.
2.
3.
4.

Right rear
Left rear
Right front

Left front

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NOTE: If the vehicle has two wheel cylinders on one brake, bleed the
upper cylinder first.

For vehicles equipped with a diagonal split section or equipped with 
ABS, follow the brake bleeding procedure recommended in the 
service information for the vehicle.

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MANUAL BLEEDING
Manual bleeding uses hydraulic pressure created by the master 
cylinder to pump fresh fluid through the brake system. This 
method is called the single stroke bleeding method.
It is extremely important when manually bleeding a brake system 
that the pedal be applied and released slowly and gently. Rapid 
pedal pumping can churn up the fluid and reduce the size of 
trapped air bubbles, making them more difficult to bleed.
Manual bleeding requires an assistant to apply and release the 
brake pedal, a bleeder screw wrench, approximately two feet of 
clear, plastic hose with an inside diameter small enough to fit 
snugly over the bleeder screws, and a clear jar partially filled

with clean brake fluid. 
Continued
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To manually bleed the brake system, follow these steps:
1. Discharge the vacuum or hydraulic power booster (if
equipped) by pumping the brake pedal with the ignition OFF 
until the pedal feels hard.
2. Fill the master cylinder reservoir with new brake fluid and 
make sure it remains at least half full throughout the bleeding 
procedure.
3. Attach the plastic hose over the bleeder screw of the first 
wheel cylinder or caliper in the bleeding sequence, and 

submerge the end of the tube in the jar of brake fluid.
See Figure 72–6.
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Figure 72–6 Bleeding brakes using clear
plastic tubing makes it easy to see air
bubbles. Submerging the hose in a container
of clean brake fluid helps ensure that all of
the air will be purged by the system.

4. Loosen the bleeder screw 
approximately one­half turn, 

and have an assistant slowly 
depress the brake pedal. Air 
bubbles leaving the bleeder 
screw will be visible in the 
hose.

5. Tighten the bleeder screw 
and have your assistant 
slowly release the brake 
pedal.
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6.

Wait at least 15 seconds to allow time for any small bubbles 
to form into larger bubbles. 

7.

Repeat steps 4 and 5 until no more air bubbles emerge from 
the bleeder.

8.

Transfer the plastic hose to the bleeder screw of the next 
wheel cylinder or caliper in the bleeding sequence, and
repeat steps 4 through 7. Continue until all brakes are bled.

NOTE: Make certain all brake components such as calipers and wheel 

cylinders are correctly installed with bleeder valve located on the highest 
section of the part. Some wheel cylinders and calipers (such as many Ford 
calipers) can be installed upside down! This usually occurs whenever both 
front calipers are off the vehicle and they accidentally get reversed left to 
right. If this occurs, the air will never be completely bled from the caliper.
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VACUUM BLEEDING
Vacuum bleeding uses a special suction pump that attaches to
the bleeder screw. The pump creates a low­pressure area at the 
bleeder screw, which allows atmospheric pressure to force brake 
fluid through the system when the bleeder screw is opened.

Vacuum bleeding requires only one technician. To vacuum bleed a 
brake system follow these steps:
1. Fill the master cylinder reservoir with new brake fluid and 
make sure it remains at least half full throughout the bleeding 
procedure.
2. Attach the plastic tube from the vacuum bleeder to the 
bleeder screw of the first wheel cylinder or caliper in the 
bleeding sequence. 
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Figure 72–7 Vacuum bleeding uses
atmospheric pressure to force brake
fluid through the hydraulic system.

If necessary, use one of the 
adapters provided with the 
vacuum in the catch bottle.

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