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The National Safety Code madates that every driver of a commercial vehicle equipped with air brakes must check push rod travel every day.

In some jurisdications, drivers must use the applied stroke method; in others, they are allowed to use the pry bar method.

Know which method you are to use to determine if the brakes are in adjustment for your province or territory.

TOOLS REQUIRED: gloves, safety glasses, wheel chocks, measuring device, device to mark push rod at fix point.

 1) Block wheels, build system pressure is at maximum pressure, and release spring brakes.

i) Air dryer purges;

ii) Confirm that needles have stopped climbing between 105 psi & 135 psi.

2) Mark all push roda at a fix point.

3) Using another person or a bit of wood, apply the brake pedal on full and hold.

i) if the trailer service brakes wrap the seat belt around the trailer hand valve.

4) At the brake chamber, measure from the fixed point to the mark on the push rod.

5) Type 30 Brake Chamber equipped with manual slack adjusters has a maximum allowable push rod travel of 1 3/4 inches (44 mm).

6) Type 30 Brake Chamber equipped with automatic slack adjuster has a maximum allowable push rod travel of 2" inches (5 cm).

5) If the push rod stroke is more that 1 3/4" inch (44 mm), the manual slack adjuster has to be adjusted.

6) If the vehicle is equipped with an automatic slack adjuster, it may have to be assessed & repaired by a qualified mechanic.


It is the size of the brake chamber that determines the maximum allowable push rod travel. The outside diameter of the brake chamber must be measured, and than you can determine the maximum allowable push rod stroke. Authorities that pull you over in the weigh scale or other places for an inspection will measure the outside diameter of the vehicle's brake chamber, and than from their chart determine maximum push rod travel. Using a two-way radio placed on your door they will go under the truck, mark the push rod at a fixed point. Over the radio, the inspection officer will tell you to make a maximum brake application. Upon applying the brakes, they will than measure to ensure that the brakes on your vehicle are within allowable limits.


Adjustment Limits for Clamp-Type Brake Chambers Brake Application (90 psi (621 kPa) to 100 psi (690 kPa) service brake application)



 Outside Diameter

 Adjustment Limit

6  None  4 ½” (115mm)  1 ¼” (32mm)
 9  None  5 ¼” (133mm)  1 3/8” (35mm)
 12  None  5 11/16” (144mm)  1 3/8” (35mm)
16 None 6 3/8” (162mm) 1 ¾” (45mm)
16L L Stamped in Cover, Stroke Tag 6 3/8” (162mm) 2” (51mm)
20 None 6 25/32” (172mm) 1 ¾” (45mm)
20L L Stamped in Cover, Stroke Tag 6 25/32” (172mm) 2” (51mm)
24 None 7 7/32” (183mm) 1 ¾” (45mm)
24L L Stamped in Cover, Stroke Tag 7 7/32” (183mm) 2” (51mm)
24LS Square Ports, Tag & Cover Marking 7 7/32” (183mm) 2 ½” (64mm)
30 None 8 3/32” (205mm) 2” (51mm)
30 DD3 (Bus/Coach) 8 1/8” 206mm 2 ¼” (57mm)
30LS Square Port, Tag & Cover Marking 8 3/32” (205mm) 2 ½” (64mm)
36 None 9” (228mm) 2 ¼” (57mm)