6.3 Adjusting Manual Slack Adjusters and Maintaining Automatic Slack Adjusters

Adjusting Manual Slack Adjusters and Maintaining Automatic Slack Adjusters

Quick Answer: Many companies do not want drivers adjusting brakes. Alberta still requires every air brake driver to understand how brakes are adjusted and, more importantly, how to recognize when they need adjustment. For a manual slack adjuster, turn the adjusting bolt so the S-cam rotates in the direction of application, since no single wrench direction is correct. Tighten until the linings contact the drum, then back off about a third of a turn. Automatic slack adjusters are maintained with full service applications at normal pressure (above 100 PSI), allowing about one second for brake lag between each application and release. Hand-adjusting an automatic unit is an emergency measure only, to get the vehicle to a repair facility.

Safety before you go under a vehicle

Checking or adjusting brakes means working near the wheels with the spring brakes released. The course is firm about this:

• Chock the wheels properly. Leaving the transmission in gear is not a substitute for chocks.
• Do not leave the ignition key in the vehicle.
• On a tractor-trailer, alternate the spring brakes so one unit always holds:
  • To check the tractor brakes: push in the yellow park control and pull out the red trailer air supply valve. The tractor springs release, and the trailer springs hold.
  • To check the trailer brakes: pull out the yellow button and push in the red one. The trailer springs release, and the tractor springs hold.

Adjusting a manual slack adjuster

Manual slack adjusters have an adjusting bolt with a square or hexagonal head and a spring-loaded lock sleeve that holds the setting. The procedure:

1. Secure the vehicle as above, with the spring brake on this axle released.
2. Push in the lock sleeve so the adjusting bolt can turn.
3. Work out which way the S-cam must roll to apply the brake. Watch the S-cam shaft, or the slack adjuster, as you turn.
4. Turn the adjusting bolt so that the S-cam rolls in the direction of application, pressing the linings toward the drum. The wrench may need to go clockwise on one brake and counter-clockwise on another, depending on how the adjuster is installed. There is no relationship between the direction of the S-cam and the direction of the wrench. A common mistake is to assume that if the cam must roll left, the wrench must turn left.
5. Watch the pushrod:
   • If the pushrod starts to pull out of the chamber, you are turning the wrench the wrong way.
   • If the pushrod moves into the chamber, the brake was previously adjusted incorrectly.
6. Tighten until the linings contact the drum, then back off, as a general guide, about a third of a turn. How much varies from person to person and with how tightly the brake was set.
7. Check the result. The free stroke should be about 1/2 inch (13 mm), and the brake must not drag.
8. Make sure the lock sleeve is back in place so the setting cannot drift.

Each student practises this twice on a foundation brake display stand in the classroom session.

Automatic slack adjusters: recognize, maintain, and emergency-adjust

Vehicles built with automatic slack adjusters cannot be retrofitted with manual ones. The five designs shown in the course differ in how they sense wear and how they are adjusted in an emergency:

Maintaining automatic slack adjusters: the full-application method

This is one of the most important and most misunderstood procedures. Automatic slack adjusters only adjust properly when the brakes stroke far enough, and normal service stops do not do that. Spring brake applications are only about half of a full service application, so setting and releasing the park brakes does not trigger the adjusters properly either.

The procedure, which you perform after the air brake pre-trip in the practical test:

1. Make sure the air pressure is in the normal range, above 100 PSI (690 kPa). Restart the engine if necessary to maintain it.
2. Make sure the spring brakes are released.
3. Press the foot valve firmly to the floor for a full service application.
4. Allow for brake lag, with about one second between each application and each release.
5. Repeat. Two full applications are made in the practical training and test. Some people call this a "six pack", meaning six full applications, but only the first few have any adjusting value.

Do not fan or pump the pedal. Short, partial strokes do nothing for the adjusters and waste air. A good habit, like scanning mirrors and gauges, is to make one full application whenever you are stopped with the drums cool and the park brakes released. The air it uses is not enough to take the pressure out of the normal range.

Emergency adjustment of an automatic slack adjuster

It must be stressed that adjusting an automatic slack adjuster by hand is not a normal procedure. The course teaches it only so that a vehicle can be taken safely to a repair facility. Doing it as regular maintenance damages the automatic slack adjusters. An automatic adjuster that is out of adjustment has a problem that needs repair.

When it is done:

• Use the wrench size and direction for that make from the table.
• The indicators that you are turning correctly are the same as for manual adjusters: the wrench should be turning the S-cam shaft in the same direction as the brake application, and the pushrod must not start pulling out of the chamber.
• For the Meritor unit, lift the pawl to back off.

Confirming adjustment on the practical test

After the full applications, you confirm that the brakes have stroked into correct adjustment by pulling the slack adjusters of both brakes on one axle. The rear trailer axle is often the easiest. The Commercial Driver's Guide trip inspection says the same thing for every brake: pull each slack adjuster by hand, or use a pry bar, to check its travel, mechanical condition, and wear. On the Air Brake Practical Test form, the full service applications are worth 15% and confirming adjustment on one axle is worth 10%. Missing both would fail the test by themselves.