Section 4.3: Measuring and Adjusting Brake Stroke

Section 4.3: Measuring and Adjusting Brake Stroke

What is Brake Stroke?

Brake stroke is the distance that the brake chamber pushrod travels outward when the service brakes are applied. It is a direct indicator of the clearance between the brake linings and the brake drum.

When the brakes are released, the pushrod sits fully retracted inside the brake chamber. When air pressure is applied to the chamber, the pushrod moves outward to rotate the slack adjuster and apply the brakes. If the clearance between the linings and the drum is correct, the pushrod only needs to travel a short distance to press the linings against the drum. However, as the linings and drum wear, or if the brake is out of adjustment, the pushrod must travel further. If the pushrod travels too far, the brake efficiency drops dramatically, or the brake may fail to work entirely. This is why measuring and monitoring the brake stroke is one of the most critical safety tasks for a commercial vehicle driver in British Columbia.

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The Mark-and-Measure Method

To accurately determine the applied pushrod stroke, drivers must use the mark-and-measure method. This procedure ensures that the measurement reflects the actual movement of the pushrod under normal operating pressures.

To perform the mark-and-measure method:

1. Secure the Vehicle: Park the vehicle on level ground. Turn off the engine and block (chock) the wheels securely to prevent the vehicle from rolling.
2. Release the Parking Brakes: Release the spring parking brakes. This is crucial because parking brakes mechanically lock the pushrod in the applied position, which prevents you from measuring the movement of the pushrod. The air pressure in the system must be at normal operating pressure (at least 90 to 100 psi).
3. Mark the Pushrod: Locate the pushrod where it exits the brake chamber. Use chalk, a marker, or a grease pencil to make a mark on the pushrod exactly where it meets the face of the brake chamber (or a specific reference point on the bracket).
4. Apply the Service Brakes: Have an assistant apply and hold the service brakes with a full application pressure of 90 to 100 psi. If you do not have an assistant, you can use a brake-pedal depressor (a tool designed to hold the pedal down) to apply the brakes.
5. Measure the Travel: With the brakes applied, measure the distance from the face of the brake chamber to the mark you made on the pushrod. This distance is the applied stroke (also known as the pushrod travel).
6. Compare to Limits: Compare the measured distance against the maximum allowable stroke limit for that specific size and type of brake chamber.

Why 90 to 100 PSI Application Pressure is Critical

When checking the pushrod stroke, applying the correct air pressure is essential. If the brakes are applied with less than 90 psi (for example, 60 psi), the force may not be enough to fully stretch the diaphragm or overcome the return springs and flex in the mechanical linkages. This will result in an artificially short stroke reading, which could give a "false pass" on a brake that is actually out of adjustment. Conversely, applying pressure higher than 100 psi does not provide a realistic simulation of standard heavy braking and can put unnecessary strain on the system during testing.

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ICBC and CVSE Brake Adjustment Limits

In British Columbia, the Commercial Vehicle Safety and Enforcement (CVSE) branch and ICBC establish strict limits for pushrod travel. These limits depend on the size of the brake chamber and whether it is a standard-stroke or long-stroke chamber.

The most common brake chamber size found on commercial vehicle drive axles and trailer axles is the Type 30 clamp-type chamber.

Standard-Stroke Type 30 Limits

For a standard-stroke Type 30 brake chamber, the maximum allowable applied stroke limit is 2.0 inches (50.8 mm). If the measured pushrod travel exceeds 2 inches, the brake is out of adjustment and the vehicle is legally defective.

Long-Stroke Type 30 Limits

Many modern vehicles are equipped with long-stroke brake chambers, which are designed to allow more travel before bottoming out. For a long-stroke Type 30 chamber, the maximum allowable applied stroke limit is 2.5 inches (63.5 mm).

Identifying Long-Stroke Chambers

It is vital that a driver does not mistake a standard chamber for a long-stroke chamber, as applying the 2.5-inch limit to a standard chamber would allow a dangerously out-of-adjustment brake to go undetected. Long-stroke chambers are identified by:

1. Trapezoidal Tags: The identification tag on the chamber is triangular or trapezoidal in shape, whereas standard chambers have round tags.
2. Square Port Bosses: The bosses where the air lines screw into the chamber are square rather than round.
3. Embossed Markings: The chamber housing may have markings or numbers indicating a long-stroke design.

Common Chamber Limits Table

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Out-of-Adjustment Defects and Driver Responsibility

When a brake pushrod travel exceeds the legal limit, it is classified as an out-of-adjustment defect. Operating a vehicle with out-of-adjustment brakes is extremely dangerous and is a leading cause of commercial vehicle runaway accidents on downgrades.

Why Excessive Stroke Reduces Braking Force

Inside the brake chamber, the flexible rubber diaphragm can only stretch so far. If the pushrod must travel too far because the brakes are out of adjustment, the diaphragm will eventually hit the bottom of the chamber housing. This condition is called bottoming out.

Once the diaphragm bottoms out, the air pressure inside the chamber can no longer push the pushrod any further. As a result, the braking force drops to zero, even if there is 100 psi of air pressure in the chamber.

Furthermore, as brake drums heat up during heavy braking, they expand. This expansion increases the distance the pushrod must travel. A brake that is barely within limits when cold can easily exceed the limits and fail when hot.

Driver Responsibility

Under BC regulations, checking and ensuring that brakes are in proper adjustment is the driver's responsibility. It is not something that can be left solely to the maintenance shop. During the daily pre-trip inspection, the driver must verify that no brake stroke exceeds the maximum limit. If even one brake is found to be out of adjustment, the driver must report the defect and have the brake serviced before operating the vehicle on a public highway.