SB-10057918-4717
TP-1538
Issued 03-15

Adjustment Procedures for Drum
Brakes on Meritor Trailer Axles

Technical Bulletin
Important Information
1Issued
TP-1538
Technical
03- Bulletin
15

Meritor automatic slack adjusters (ASAs) should not need to be
manually adjusted in service. If the chamber push rod clevis is
installed correctly, the ASAs should not have to be adjusted to
correct excessive push rod stroke. The excessive stroke may be an
indication that a problem exists with the foundation brake, ASA,
brake actuator or other system components.
Meritor recommends troubleshooting the problem, replacing
suspect components and then confirming proper brake operation
prior to returning the vehicle into service.
In the event that a manual adjustment must be made (although not a
common practice), a service appointment and full foundation brake,
ASA, and other system component inspection should be conducted
as soon as possible to ensure integrity of the overall brake system.
For Meritor brake adjustment, refer to the brake adjustment tables
in this technical bulletin. For non-Meritor brake adjusters, refer to
the brake manufacturer’s service procedures.

Hazard Alert Messages
Read and observe all Warning and Caution hazard alert messages in
this publication. They provide information that can help prevent
serious personal injury, damage to components, or both.

ASBESTOS AND NON-ASBESTOS
FIBERS WARNING
Some brake linings contain asbestos fibers, a cancer and lung
disease hazard. Some brake linings contain non-asbestos
fibers, whose long-term effects to health are unknown. You
must use caution when you handle both asbestos and
non-asbestos materials.

WARNING
To prevent serious eye injury, always wear safe eye protection
when you perform vehicle maintenance and service.

Park the vehicle on a level surface. Block the wheels to
prevent the vehicle from moving. Support the vehicle with
safety stands. Do not work under a vehicle supported only by
jacks. Jacks can slip or fall over. Serious personal injury and
damage to components can result.
Before you service a spring chamber, carefully follow the
manufacturer’s instructions to compress and lock the spring to
completely release the brake. Verify that no air pressure
remains in the service chamber before you proceed. Sudden
release of compressed air can cause serious personal injury
and damage to components.
You must disengage a pull pawl before rotating the manual
adjusting nut, or you will damage the pawl teeth. A damaged
pawl will not allow the slack adjuster to automatically adjust
brake clearance. Replace damaged pawls before putting the
vehicle in service.

How to Obtain Additional Maintenance,
Service and Product Information
Refer to Maintenance Manual 4, Cam Brakes and Automatic Slack
Adjusters; Maintenance Manual 14, Trailer Axles; and Parts Catalog
PB-8857, Brake, Trailer Axle and Wheel Attaching Parts. To obtain
these publications, visit Literature on Demand at meritor.com.

How an Automatic Slack Adjuster
Works
When you install an automatic slack adjuster, you set the brake
chamber stroke measurement, which is the correct shoe-to-drum
clearance. When linings wear, this clearance increases, and the air
chamber push rod must travel farther to apply the brakes.
When this happens, the slack adjuster will automatically adjust
during the return stroke to maintain the correct shoe-to-drum
clearance. If the air brake chamber push rod stroke is within limits
during operation, no adjustment occurs.

Adjustment

4.

Measure Free Stroke
When you perform preventive maintenance procedures on an
in-service brake, check both the free stroke and adjusted chamber
stroke. Refer to the procedures in this section.
Free stroke sets the clearance between the linings and drum. The
in-service free stroke may be slightly longer than 0.5-0.625-inch
(12.7-15.9 mm) specified in this procedure. This is acceptable if the
adjusted chamber stroke is within the limits shown in Table A and
Table B. If partial rotational contact is present at 0.625-inch
(15.9 mm) free stroke, the drum may need remounting.
1.

Disengage a pull pawl. Use a screwdriver or tool to pry the pull
pawl at least 1/32-inch (0.8 mm) to disengage the teeth.

2.

Use a wrench to turn the adjusting nut COUNTERCLOCKWISE
until the brake shoes contact the drum. Figure 1. Then back off
the adjusting nut in the opposite direction 1/2 turn for drum
brakes.

Use a pry bar to move the slack adjuster and position the
linings against the drum, brakes applied. Measure the same
distance again while the brakes are applied. The measurement
you obtain is Y in Figure 2.

CAUTION
Do not set free stroke shorter than 0.5-0.625-inch
(12.7-15.9 mm) for drum brakes. If the measurement is too
short, linings can drag. Damage to components can result.
5.

Subtract X from Y to obtain the in-service free stroke. The
measurement must be 0.5-0.625-inch (12.7-15.9 mm) for
drum brakes. Figure 2.
앫 If the free stroke measurement is not within
specification: Turn the adjusting nut 1/8 turn in the
direction shown in Figure 3 and check the free stroke
again. Continue to measure and adjust the stroke until the
measurement is within specification.

Figure 3

Disengage a
pull pawl.

Figure 1

SHORTEN
STROKE

Disengage pawl.

LENGTHEN
STROKE

4000373b
Figure 3

4000371e
Figure 1

3.

Measure the distance from the center of the large clevis pin to
the bottom of the air chamber while the brake is released. The
measurement you obtain is X in Figure 2.

6.

Re-engage the pull pawl by removing the screwdriver or tool.
The pull pawl will re-engage automatically.

7.

If the brakes have spring chambers, carefully release the
springs. Test the vehicle before you return it to service.

Commercial Vehicle Safety Alliance
(CVSA) Guidelines

Figure 2

MEASURE FREE STROKE
“Y”
“X”

Measure Push Rod Travel or Adjusted
Chamber Stroke
FREE STROKE = Y MINUS X
Drum brake free stroke must be
0.5-0.625" (12.7-15.9 mm).

Use the following procedure to check in-service push rod travel or
adjusted chamber stroke on truck and tractor brakes.
1.

4000372c
Figure 2

TP-1538
Issued 03-15
Page 2

The engine must be OFF. If the brake has a spring chamber,
follow the manufacturer’s instructions to release the spring.
Verify that no air pressure remains in the service section of the
chamber.

Copyright Meritor, Inc., 2015

(16579)
Printed in USA

2.

Verify that pressure is 90-100 psi (621-689 kPa) in the air
tanks. Determine the size and type of brake chambers on the
vehicle.

3.

With the brakes released, mark the push rod where it exits the
chamber. Measure and record the distance. Have another
person apply and hold the brakes on full application. Figure 4.
Hold the ruler parallel to the push rod and measure as carefully
as possible. A measurement error can affect CVSA
re-adjustment limits. CVSA states that “any brake 1/4-inch or
more past the re-adjustment limit, or any two brakes less than
1/4-inch beyond the re-adjustment limit, will be cause for
rejection.”

Figure 4

Step 1

Mark push rod here
to measure stroke.

Step 3

Refer to Table A or Table B to verify that the stroke length is
correct for the size and type of air chambers on the vehicle.
앫 If the adjusted chamber stroke is greater than the
maximum stroke shown in Table A or Table B: Diagnose
and correct the problem.

Table A: Standard-Stroke Clamp-Type Brake Chamber Data

Type

Outside Diameter
(inches)

Brake
Adjustment Limit
(inches)

24

7-7/32

1-3/4

30

8-3/32

2

36

9

2-1/4

Table B: Long-Stroke Clamp-Type Brake Chamber Data

• Spring brakes
released
• Service brakes
not applied

6.

Step 2

Type

Outside Diameter
(inches)

Brake
Adjustment Limit
(inches)

20

6-25/32

2.0

24

7-7/32

2.0

24*

7-7/32

2.5

30

8-3/32

2.5

STROKE

90-100 psi (620-690 kPa)
in air tank — engine OFF

Alternate Method to Measure Push Rod
Travel or Adjusted Chamber Stroke

• Spring brakes
released
• Service brakes
applied

4000374b

Figure 4

4.

Measure the push rod travel or adjusted chamber stroke from
where the push rod exits the brake chamber to your mark on
the push rod. Measure and record the distance. Figure 4.

5.

Subtract the measurement you recorded in Step 3 from the
measurement you recorded in Step 4. The difference is the
push rod travel or adjusted chamber stroke.

Meritor Heavy Vehicle Systems, LLC
2135 West Maple Road
Troy, MI 48084 USA
866-OnTrac1 (668-7221)
meritor.com

* For 3" maximum stroke type 24 chambers.

Use the CVSA procedure, except in Steps 3 and 4, measure the
distance from the bottom of the air chamber to the center of the
large clevis pin on each of the brakes.

CVSA North American Out-of-Service Criteria
Reference Tables
Information contained in Table A and Table B is for reference only.
Consult the CVSA Out-of-Service Criteria Handbook for North
American Standards, Appendix A. Visit their website at http://
64.35.82.7/ to obtain the handbook.

Information contained in this publication was in effect at the time the publication was approved for printing and is subject to
change without notice or liability. Meritor Heavy Vehicle Systems, LLC, reserves the right to revise the information presented or
to discontinue the production of parts described at any time.
Copyright 2015
Meritor, Inc.
All Rights Reserved

Printed in USA

TP-1538
Issued 03-15
(16579)