Always ensure the vehicle is securely chocked before releasing the parking brake.
Cage the spring brake or use a minimum hold off pressure of 90 psi (6 bar).
Automatic Slack Adjusters should maintain a constant stroke of the actuator.
At regular service intervals, appropriate to the application, the following checks should be made:
Apply the brake. Observe the stroke length of the actuator.
The stroke should not exceed 2/3 of the maximum stroke of the actuator. If the stroke is more than 2/3 of maximum stroke, carry out the annual check.
Examine the foundation brake system for seized brake shoes and cam rollers. If no other causes of excess stroke are found, replace the slack adjuster.
Check the anchor point and the condition of the control arm. If the anchor point is loose, re-fit it.
If the control arm is bent or cracked replace the slack adjuster.
Check the condition of the camshaft journal and bush. If there is more than O.5mm gap replace the camshaft and bush.
Webb Automatic and Self-setting Automatic Slack Adjusters should be lubricated every 10,000km.
The following check should be made annually for truck and trailer applications. For city bus applications it should be carried out every quarter.
Using a torque wrench, rotate the hex nut in anti-clockwise direction:
There should be an audible clicking sound.
Repeat the operation to ensure the clutch is engaged.
The torque required to turn the hex nut must be 18Nm.
If there is no clicking sound or the torque required is less than 18Nm replace the slack adjuster.
Using a socket wrench or socket mounted indicator, rotate the hex nut 270° in anti-clockwise direction.
Apply the brake 5 times.
The wrench or indicator will move in a clockwise direction on each application of the brake.
If the wrench or indicator does not move replace the slack adjuster.
Re-set the brake by putting a socket wrench on to the hex nut and turning until the brake linings touch the brake drum.
De-adjust the brake by turning the socket wrench in the opposite direction by 1/2 a turn.
Uncage the spring brake and apply the parking brake prior to removing the chocks from the vehicle.
Note that Webb Automatic and Self-setting Automatic Slack Adjusters cannot compensate for other defects within the braking system. Worn or missing return springs, loose wheel bearings, defective actuators, worn camshaft journals, camshaft bushes or camshaft rollers and worn anchor pins must all be rectified for the brake system to function properly.