Commercial drivers are required by law to ensure their brakes are in working order and properly adjusted. The mark-and-measure method is considered the only reliable means of measuring brake stroke. That is how roadside inspectors measure brake adjustment.
(We’ll demonstrate alternate methods later in this video.)
We are going to measure the pushrod travel on a trailer brake, so get out and place your wheel chocks first. Start the truck then release the trailer brakes only. Build the air pressure up to between 90 and 100 psi, then shut the truck off.
To determine the adjustment limit, or maximum allowable stroke length, we must first determine the size and type of brake chamber. In this case, the size and type are stamped into the housing. It’s a Type 3030, with a 2.5-inch stroke. Note, that is not the adjustment limit; it indicates the chamber’s maximum possible stroke. Referring to the chart, a Size 30 chamber with no special markings has an adjustment limit of two inches.
Measure the applied stroke of the pushrod by first marking the pushrod precisely at the face of the chamber or some suitable fixed point with the brakes released. Then, with the brakes fully applied, measure the distance the mark is from the face of the brake chamber. I this case, I placed a piece of tape on the pushrod at the face of the chamber. You’ll need someone to help you apply the brakes for this.
In this case, the length of the pushrod stroke is 1-1/2 inches. That’s under the two-inch limit for that type of chamber, so that brake is good to go.
Repeat that process for all the brakes on the truck.
There are other recognized ways of checking brake stroke, but they are not as reliable as mark-and-measure, for several reasons. I’ll explain why in a moment.
Measuring free stroke
Drivers can also measure free stroke, which is the distance the pushrod travels from its fully retracted position until the brake shoes contact the brake drum. Use a wrench or a prybar to increase your leverage. Sometimes the return springs are pretty stiff.
In this case, the free stroke measured 1-1/4 inches. That’s well within the two-inch limit, so it’s good to go too. Or is it?
How do you account for that quarter-inch difference between the applied stroke and the free stroke measurement? With a 100 psi, full-pressure application, a 30 square-inch brake chamber exerts 3,000 pounds of force on the push rod. That causes some deformation in the brake components, and it even expands the brake a bit and compresses the brake linings.
If a free stroke measurement showed 1-3/4 inches, a full-pressure applied stroke measurement might be at or over the two-inch stroke limit. Free stroke measurement can give you a false sense of security.
Other ways to measure stroke length
Another accepted by not terribly reliable method is to measure the stroke length with the parking brakes applied.
The trouble here is the parking brake springs exert only about the equivalent of a 60-psi application. That means the pushrod won’t travel as far as it will under a full 100-psi application. Again, this can give you a false sense of security about your brake adjustment.
The third and least reliable method is to eyeball the angle between the pushrod and the slack adjuster. If the brake was set up properly to begin with, the angle will be 90 degrees or more at the stroke limit under a full pressure application. There are simply too many variables at play to make this a reliable way of ensuring the brakes are within their stroke limit.
If you want to be sure your brakes are properly adjusted, mark-and measure at full application pressure is the only way to go. If you use one of the other methods, be aware that under a full application, the stroke will probably be longer. If you’re close to the limit with one of the alternate methods, have the brakes checked by a mechanic.