Slack Adjusters and Brake Adjustment on Air Brakes
Out-of-adjustment brakes are among the most common inspection findings and among the most consequential. Here is how the system works.
Brake adjustment is consistently one of the most common things found wrong on a roadside inspection, and it is also one of the more consequential. A brake out of adjustment does not stop doing anything, it just does less of it, and you do not find out how much less until you need all of it.
Here is what is actually happening.
How the system works
Air pressure pushes a rod out of a chamber. That rod turns the slack adjuster, which turns the S-cam, which pushes the shoes against the drum. The distance the rod travels before the shoes contact is pushrod stroke, and that is what adjustment controls.
As linings wear, the shoes have further to travel. Something has to take that up, or stroke keeps increasing until the chamber runs out of travel and the brake stops applying effectively.
Automatic slack adjusters are not automatic forever
Nearly everything on the road runs automatic slack adjusters, and the assumption is that they handle it. They do, until they do not.
An automatic adjuster that has failed leaves the brake out of adjustment while everyone assumes it is fine. That is worse than a manual adjuster, because at least a manual one gets checked.
This is the important part: automatic adjusters still get inspected. Finding one out of adjustment means the adjuster has a problem, and manually adjusting it is treating the symptom. The adjuster itself needs attention.
Checking stroke
Marking the pushrod with the brakes released and measuring how far it travels on a full application tells you the stroke. Compare against the limit for that chamber size and type.
It has to be done with the system at pressure and the brakes fully applied, and there is a procedure. Guessing by looking at the angle does not work.
If you find one brake out of adjustment, check them all. Brakes work as a system, and one that is doing less means the others are doing more, which is why one dragging or one out of adjustment shows up as uneven wear across the truck.
Why it matters beyond the violation
A brake out of adjustment contributes less braking force. The other brakes take up the difference, which means they work harder, run hotter and wear faster.
On a loaded truck on a grade, that imbalance is what turns a controlled descent into a problem. Brakes that are all doing their share handle heat far better than a system where three are compensating for one.
What else affects it
- Worn linings, which increase stroke as they thin
- Cracked or worn drums
- Worn S-cam bushings, which allow movement that does not go into pushing the shoes
- Seized or corroded cam rollers
- A failed or seized automatic adjuster
- Chamber diaphragm failure, which loses force
- Improper installation after a component change
Heat and drums
Drums crack from heat, and heat comes from brakes working harder than they should. A cracked drum is an out-of-service item and it is also a consequence of something upstream.
Blue discoloration on a drum means it has been hot. That is worth investigating rather than noting.
Inspection habit
Brake adjustment belongs in a real inspection rather than a glance. It requires the system charged, a full application and a measurement, which is more than a walkaround provides.
It is also exactly the kind of thing that gets scheduled and skipped, and then shows up on a roadside report.
Trailer brakes
Everything above applies to trailer brakes, and trailers get checked far less because nobody owns them. A trailer with brakes out of adjustment puts more work on the tractor, which shows up as tractor brakes wearing fast.
If the tractor is going through brakes quickly, the trailer is worth checking before assuming the problem is on the truck.
Air pressure and application
Adjustment is measured with the system at full pressure and the brakes fully applied. Measuring at partial pressure gives you a number that means nothing, which is how brakes that were 'checked' end up out of adjustment on an inspection.
If you want the brakes checked properly, adjustment measured and the adjusters themselves looked at, call (850) 495-0366. Dispatch answers 24 hours and we come to the truck or the yard anywhere in the panhandle.