How to adjust air brakes

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What is Brake Adjustment?

Pushrod stroke length exceeds its limit with continuous use of brakes. Bringing pushrod stroke back to the limits is called brake adjustment.

Why the brake adjustment is required?

As a road user, I am responsible for my safety. I’m safe on the road if my vehicle’s brakes are fully functional. Air brake systems are the cheapest and best way to control big vehicles on the road. The air brake system is used in all air brakes. Therefore air brakes should remain adjusted all the time to stay safe on roads.

How to adjust air brakes?

Applied pushrod stroke measurement tells the brakes are adjusted or not. To adjust brakes determine the applied pushrod stroke and match with the limits in a table below.

Pushrod stroke length limits

The length of the applied pushrod stroke depends on the size and type of the brake chamber. The most used brake chamber size in trucks is 30. The table below has the limit of brake chamber size and allowed pushrod stroke.

Clamp type brake chambers limits at 90psi to 100psi
Size Markings Outside Diameter Adjustment limit
6 NONE 4 1/2″(115mm) 1 1/4″(32mm)
9 NONE 5 1/4″(133mm) 1 3/8″(35mm)
12 NONE 5 11/16″(144mm) 1 3/8″(35mm)
16 NONE 6 3/8″(162mm) 1 3/4″(45mm)
16L L Stamped in Cover,
Stroke Tag
6 3/8″(162mm) 2″(51mm)
20 NONE 6 22/32″(172mm) 1 3/4″(45mm)
20L L Stamped in Cover,
Stroke Tag
6 22/32″(172mm) 2″(51mm)
24 NONE 7 7/32″(183mm) 1 3/4″(45mm)
24L L Stamped in Cover,
Stroke Tag
7 7/32″(183mm) 2″(51mm)
24LS Square port, Tag &
Cover Marking
7 7/32″(183mm) 2 1/2″(64mm)
30 NONE 8 3/32″(205mm) 2″(51mm)
30 DD3 (Coach, Bus) 8 1/8″(206mm) 2 1/4″(57mm)
30LS Square port, Tag &
Cover Marking
8 3/32″(205mm) 2 1/2″(64mm)
36 NONE 9″(228mm) 2 1/4″(57mm)

How to measure applied pushrod stroke?

There are two methods to measure applied pushrod stroke.

Method 1. Mark and measure.

This is the most reliable way to measure the applied pushrod stroke. Step by step procedure to measure applied pushrod stroke is as follows.

  • Apply the wheel chock and put the transmission in neutral.

    air brake chamber
    Brake chamber
  • Ensure your air pressure is above 90psi and release the spring brakes.
  • Mark the pushrod on each brake chamber, where it meets the brake chamber.
  • Now come back into the vehicle, turn off the engine and apply service brakes (make sure your air pressure stays between 90 to 100psi after application of service brakes).
  • Go outside and measure the applied pushrod length from your marking to the brake chamber. Repeat it for each brake chamber.
  • Now find out the size and type of brake chamber and measure its diameter.
  • Release service breaks and apply spring breaks.
  • Match your measurement with the corresponding lengths in brake adjustment limits chart.
  • If your measurement exceeds the numbers given in the chart, then your brake is out of adjustment.
  • Your brakes are adjusted if your measurement is equal to or less with the numbers given in the chart.

Pre-trip Inspection

Method 2. Mark Pushrod permanently

This is the second method to measure the applied pushrod stroke.
Adjust the brake once and mark the pushrod exactly where it meets the brake chamber. Best to put a permanent marking on the rod. Just look at the markings, to determine whether brakes are out or in the adjustment.
Brakes are adjusted if there is no gap between markings and brake chamber.

Break Adjustment Indicators

Brake adjustment indicator
Brake adjustment indicator.

Brake adjustment indicator is a small mechanism and it is attached to the brake chambers.
Brakes are adjusted if your stroke indicator knob is within the yellow posts. Otherwise, the brakes are out of the adjustment.

Slack Adjusters

It connects pushrod to the braking mechanism. Two types of slack adjuster are available.

slack adjuster tool, slack adjuster tool napa
Adjuster Tool

Manual slack adjustors.

Manual slack adjusters were used in tractor trailers before 1994.

Automatic Slack Adjuster Wrench slack adjuster tool long
Automatic Slack Adjuster Wrench

A certified technician can adjust brakes, still, you can try the following methods.

Automatic Slack Adjuster

The automatic adjuster does not allow brakes to go out of adjustment. Automatic slack adjusters need proper lubrication.

How to adjust brakes on a semi-trailer?

To adjust brakes on a semi-trailer,

  • Make sure the trailer brakes released and wheel chocks on.
  • Locate the slack adjusting mechanism and tighten the attachment bolt with 9/16 wrench.
  • Watch the S Cam movement and brake shoes moving towards the brake drum.
  • Now loosen the bolt 1/4 turn, now try to move the trailer, if it is dragging, loosen the bolt again.
  • Rear drum brakes over adjust if we overtight it.
  • See the stroke length now. If your brake chamber size is 30 then the rod length should not exceed 2″.
S-Cam Brake Slack Adjuster Tool, slack adjuster tool
S-Cam Brake Adjuster Tool

How often to adjust drum brakes?

Bendix is the biggest manufacturer of brakes. They recommend adjusting after every 8000 miles or 300 hours.

How to adjust air brakes sitting in the truck. 

This is another way to adjust the breaks with the automatic slack adjuster is by applying and holding the service brake for 10 seconds. Release and repeat this process 6 times. Your brakes will be adjusted if automatic slack adjusters lubricated and working properly.

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