Regional Sales Manager, Crewson Brunner

Automatic brake adjusters (ABA's), or automatic slack adjusters (ASA's), are here to stay. They've been mandatory on all new air brake vehicles since October 1994. But there are some things that need to be discussed about why you are adjusting your automatic brake adjuster.

There are five automatic brake adjuster manufacturers: Crewson Brunner, Meritor Automotive, Allied Signal (Bendix), and Haldex Midland. Each design has its own features and benefits and if you ask any of them, the topic will be a hot one. There are only two adjusting concepts among all five suppliers: Clearance sensing; or, Stroke sensing. Each design has gone through improvements in performance and durability over the years.

Q: When is the brake adjustment completed?

A: On the return air or the apply stroke.

Clearance sensing designs (Crewson Brunner, Haldex, Gunite and Bendix) maintain a specified shoe-to-drum running clearance determined initially by the clutch mechanisms. This running clearance ranges from .020 to .030 in air gap between the friction material and the drum. The internal clutch senses the force that develops as the brake is applied and the friction material contacts the drum. If brake adjustment is needed, the clutch is engaged to make the adjustment. If no brake adjustment is needed, the clutch will partially or fully disengage depending on the brand of the ASA. Clearance sensing provides a more consistent running clearance in all braking modes. Crewson Brunner is the only design that "fully" separates the clutch halves to reduce wear on these internal components.

Only Meritor uses a stroke-sensing design. As the push rod moves to apply the brake, the ASA senses the amount of push rod travel and if it is past the readjust limit, it makes brake adjustments accordingly.

ASA is accomplished in two ways:

1. On the return stroke (Crewson Brunner, Haldex, and Meritor)
2. On the apply stroke (Gunite and Bendix)

All auto-slacks adjust the brakes during the low air pressure portion of a brake application (usually 20 P.S.I. or less). Each method can be affected by wear in the foundation brake componentry. But it's important that the ABA adjust on the return stroke at the end of the brake release. This enables all connecting parts; air chamber, shoe return springs and other brake related parts, whether new or worn, to work together.

If the ABA adjusts on the apply stroke, it may have to turn the adjusting mechanism when the friction material is in contact with the drum making it difficult to achieve proper adjustment.

A: The main reason is because people don't understand how ASA's function. Other reasons are:

• They don't manually cage the spring brake properly during installation.
• The push rod is not cut to the correct length.
• They don't change the clevis. Every ASA manufacturer has their own specific clevis.
• They don't use the template to set the slack up during installation.

All instructions indicate that you should manually cage the spring brake. This lets the service diaphragm retract as far as possible with the springs locked down. All the push rods need to be the same length! The push rod length is important because the ABA's will not be set up the same, and you have unbalanced brakes from wheel to wheel. This can cause premature and uneven wear from side to side, and even wheel lockup.

Clevises are not interchangeable! You must use the clevis provided from that particular manufacturer. Yes, you can make them fit, but, the slack will not perform properly. All of the manufactures require set up a procedure or a template to "Setup" the clevis before ASA installation.

If you follow the instructions included in the box, the amount of time and labor spent working on automatic brake adjusters will be at a minimum. ABA's are not going to end your brake related work. Include them into your preventive maintenance schedules for inspection and lubrication just like any other component. ABA's may reduce certain brake maintenance expenses by lowering the amount of labor spent adjusting manual slacks but ABA's do not reduce the need to perform other brake maintenance like replacing cam bushings, spider bushings, friction material and brake springs.

If you are adjusting your automatic brake adjusters, you need to stop and ask yourself one question! "If this is supposed to be AUTOMATIC, then why am I doing this"???

Incorrect installation of ABA's is often a problem on new vehicles. Never take for granted that the installation is correct just because they came from the OE factory. If you are adjusting your slacks on a new vehicle, chances are they were not properly installed at the factory. The way to correct this problem is to use the manufacturer's instruction sheet and manufacturer template and take them off and reset the clevis before reinstalling the ASA. The time spent checking the setup will be paid back many times over with years of dependable service.

Source: School Bus Technician, September, 2000