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Vehicles made after 1994 have automatic slack adjusters that adjust themselves during full brake applications and should only require manual adjustments during installation. However, all brake systems, whether manual or automatic slack adjusters, should receive regular maintenance checkups.

When you sign the trip inspection form, you need to check the brakes to make sure they are in adjustment. This means marking and measuring the brake stroke which is part of the trip inspection. While there are some short-cut methods to determine brake adjustment but they will not be accurate enough to pass a DOT inspection.

What does it do?

It adjusts and takes up the slack in the stroke as the friction surfaces wear away. In a “normal” U.S. highway environment a manual slack adjuster would probably need adjustment approximately every 10,000 miles. But since there are no such things as normal truck brakes, slack adjusters are designed to be adjusted. You cannot go by the mileage interval, like an oil change, you have to make the adjustment when it’s required.

Where is it?

  • On an air-brake system, the slack adjuster is what is used to adjust the brakes. It is located on the axle housing on the air canister near the wheel.
  • On drum brakes, it’s between the pushrod and the S-cam.
  • On disc brakes, it is between the pushrod and the power screw.

How to do it?

You can release the spring brakes and build the system pressure to 100 psi, make a full pressure application and note the pressure drop. According to some a 2-psi per axle drop in system pressure, or 10-psi on a 5-axle unit, is acceptable. The problem is, how do you know that two brakes are adjusted well below the limit and three are over-stroking by an eighth of an inch? You cannot tell, but an inspector can.

Some say if the slack adjuster is at no more than a 90-degree angle to the push rod when the spring brakes are applied, the adjustment is good. At the same time, many believe that if, when using brake stroke indicators, the pointer is inside the markers when the spring brakes are applied than the adjustment is good. Unfortunately,  neither is true, and neither way will measure the brake stroke accurately.

Manual adjusters

  • Under certain conditions, manual slack adjusters could get close to the adjustment limit in a short time.
  • Improper adjustment often a matter of operator negligence

Automatic adjusters

  • Automatic slack adjusters do a great job of keeping stroke within limits overall, but a lot are found to be out of adjustment during roadside brake inspections.
  • Normally don’t require manual readjustment.
  • When they go out of adjustment something else is usually happening.
  • If a brake is over-stroking with an automatic adjuster there is a problem with the brake or adjuster.
  • Adjusting it won’t really fix the problem, manually adjusting at this point will only temporarily fix the problem.
  • Faulty auto slacks are rare.
  • The issue is likely an improper installation, faulty or worn components in the brake system.
  • Lack of use:
    • If you are a good driver who manages speed, keeps a safe distance and coast to traffic lights you likely never make an application more than 15 or 20 psi, the brakes wear naturally and the auto slacks don’t compensate.
    • Solve the problem by weekly making 6 full pressure applications once a week or to make the adjuster turn over and visually check the stroke before leaving the yard.

A large number of trucks are taken out of service due to brake defects and many of those are due to issues with adjustment. You may worry about getting a ticket if your brakes are not adjusted correctly, but the result could be far worse than just getting a ticket. Please keep this in mind and remember to be safe rather than sorry.

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