!

WARNING: Do not remove the spring chamber clamp ring (clamp closest to the release bolt) as serious injury may result
from sudden release of the power spring.

Fig. 1
“Cut”
Note: Product information and specifications subject to change without notice.

Fig. 2
“Rotate”

147

Fig. 3
“Insert”

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TECHNICAL

MIXING STANDARD AND LONG STROKE CHAMBERS

1.0

SCOPE:

Installing and replacing brake chambers with different rated strokes – specifically long stroke
versus standard stroke.

2.0

RECOMMENDATION:

MGM Brakes recommends replacing chambers with the same stroke rating across an axle. When
servicing a truck/trailer equipped with either standard stroke or long stroke chambers, it is crucial
that replacement of the chambers be like for like. For example: if a 3” (76.2 mm) stroke chamber
is to replace a 2.5” (63.5 mm) stroke chamber, both chambers on that axle must be upgraded to
3” (76.2 mm) chambers.

CAUTION:
When upgrading from standard stroke to long stroke chambers, be sure there is:

1) adequate clearance for the increased stroke of the pushrod, yoke, and slack adjuster at full
stroke

2) adequate clearance of the longer chamber at the worst case suspension and steering
articulation.

TECHNICAL

Approval from the original vehicle manufacturer for proper clearance is recommended.

3.0

BACKGROUND:

The heat generated by the contact of the lining against the drum, especially during hard or
continuous braking situations, causes the drum to “grow”, or expand. The ability of a 2.5” (63.5 mm)
“standard” stroke chamber to provide adequate pressure of the lining against the ever-expanding drum
is limited by the stroke and performance characteristics of the brake. As the 2.5” (63.5 mm) stroke
chamber exceeds 2” (50.8 mm) of travel, the operator will begin to sense the “brake fade” phenomenon,
which will become more prevalent as the chamber moves closer to it’s maximum travel position, at which
point the brake will cease to provide any braking capability.

However, the performance and operational characteristics of the MGM 3-inch “long stroke” chamber
provides braking force beyond the point where the 2.5” (63.5 mm) “standard” stroke chamber ceases to
provide braking. The 3” (76.2 mm) “long stroke” chamber provides the same force - output at 2.5” (63.5
mm) of stoke as provided by the “standard” chamber at 2” (50.8 mm) of stroke.

Therefore, because of the differences in the operational characteristics of the standard and long stroke
chambers, when they are mixed across an axle, an imbalance may exist in the stopping capability of the
vehicle at strokes beyond the recommended readjustment limit of the standard stroke chamber.
This may result in the vehicle being “pulled” toward the side of the “long stroke” chamber, which is doing
the majority of the braking. This may go unnoticed by the driver during “normal” brake applications,
but could cause vehicle instability during “emergency” braking.

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148

Note: Product information and specifications subject to change without notice.

FIELD REPLACEMENT
FIELD REPLACEMENT OF CHAMBERS
EQUIPPED WITH “WELDED” YOKES
Every day, more and more vehicles are being manufactured with 3” (76.2 mm) “long stroke” spring brakes and service chambers with
the yoke “welded” to the service push rod.
Because the “welded” yoke is permanently attached to the push-rod, it is virtually impossible to remove and re-use it. Therefore,
unless the non-pressure housing (mounting base), push-rod, and/or yoke have been damaged, or are severely worn, MGM Brakes
recommends replacement of the single/piggyback unit only.
However, if replacement of the combination/tandem unit is required, it is recommended that a unit equipped with a “welded” yoke be used.
Y - DIM.

Welded
Yoke

Forged Yoke
& Jam Nut

Units manufactured with a “welded” yoke have a preset “Y”
dimension, measured from the bottom of the NPC to the
center-line of the yoke pin, of 2.25” (57.2 mm). Because the “Y”
dimension is relatively short, the corresponding “X” dimension,
measured from the bottom of the NPC to the end of the rod, will
also be short, at approximately ¾ of an inch (19.1 mm).
Consequently, if a replacement unit with a “universal” all-threaded
cut-to-fit push rod is used, the “stack-up” of the yoke and jam
nut will exceed the “X” dimension by approximately 1/8th of an
inch (3.2 mm).
In some cases, this will prevent the unit from achieving zerostroke, or, if the push rod opening in the NPC is large enough,
the jam nut will protrude into the NPC and may damage the
protective stone shield.

Spring brakes and service chambers must fully retract to zero-stroke in order to completely release the foundation brakes so the lining
no longer contacts the drum, and to allow the automatic slack adjusters to properly operate and adjust the stroke.
Therefore, do not use a replacement unit with a universal all-threaded cut-to-fit push rod to replace a unit with a “welded” yoke.
NOTE: Be sure to always replace “like for like”. Do not mix 2.5” (63.5 mm) “standard stroke” chambers with 3” (76.2 mm)
“long stroke” chambers across an axle. Reference MGM Technical Bulletin on page 146.

Note: Product information and specifications subject to change without notice.

149

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TECHNICAL

1.3

PERFORMANCE CHART - SPRING BRAKE CHAMBERS
SAE
DOUBLE DIAPHRAGM SPRING BRAKE CHAMBERS
in-lbs. of Torque Applied @ 1.25”
Stroke by S-Cam Camshaft w/Slack
Adjuster Length
of (in)**

Spring
Chamber
Size

Rated
Service
Stroke
(in)

Vol.
Displ. @
100psi
(in3) (a)

Hold-Off
Pressure
Required
(psi) (b)

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

5.00

5.50

6.00

6.50

TR2024

24

2.25

76.2

78

1280

1210

1150

1080

1000

840

-

-

-

6050

6655

7260

7865

TRxx24HD

24

2.25

76.2

78

1480

1396

1320

1250

1177

1074

-

-

-

6980

7678

8376

9074

TRxx24L

24

2.50

76.2

78

1280

1210

1150

1080

1000

840

620

-

-

6050

6655

7260

7865

TRxx30

30

2.50

101.8

64

1510

1420

1339

1257

1172

1084

994

-

-

7100

7810

8520

9230

TRxx30HD

30

2.50

103.2

68

1681

1580

1488

1396

1303

1211

1119

-

-

7900

8690

9480

10270

TRxx30LP3HD

30

3.00

115.4

68

1681

1580

1488

1396

1303

1211

1119

1026

934

7900

8690

9480

10270

TRxx30LP3

30

3.00

115.4

63

1519

1435

1352

1270

1187

1098

1009

919

837

7175

7893

8610

9328

TR3036

36

2.50

121.9

66

2050

1960

1867

1774

1679

1584

1487

-

-

9800

10780

11760

12740

TR3636

36

3.00

138.7

68

2050

1960

1867

1774

1679

1584

1487

1390

1280

9800

10780

11760

12740

LTRxx30T

30

2.50

104.8

70

1510

1420

1350

1280

1210

1145

1070

-

-

7100

7810

8520

9230

LTRxx30L3

30

3.00

108

71

1615

1525

1435

1350

1260

1160

1050

935

-

7625

8388

9150

9913

MODELS (d)

Spring Brake Force-Output (Pounds-Force) @ in. of Stroke (c)