An important part of disc brake maintenance is that for those working with disc brake callipers, check and adjust is the rule.
Whenever new callipers or brake pads are installed, the calliper self-adjust operation must be checked. If that is working correctly, the callipers must then be set to the correct operating clearance.
These checks and adjustments need to be carried out with the unit prepared for service (refer to applicable OEM guidelines) and the brake chamber, brake pads and pressure plate in place.
NOTE: The following instructions apply to WABCO and Knorr-Bremse air disc brake systems commonly used by Hendrickson suspensions and other some suspension manufacturers.
- Carefully remove the adjuster plug from the back of the calliper. Ensure that you do not damage the surrounding adjuster boot.
- Inspect adjuster, adjuster boot and plug for wear, damage or deterioration.
- Turn the adjuster half a turn clockwise on WABCO callipers with 8-mm deep offset spanner and anticlockwise on Knorr-Bremse callipers using a 10-mm deep offset box spanner or a standard ring spanner paired with the adaptor socket from the calliper service tool kit. Do not use an open-ended spanner.
- Leave the spanner on the adjuster.
- Get an assistant to gently apply the brakes five times while you watch the spanner for movement.
- If the adjuster is operating correctly, the spanner will move anticlockwise on WABCO callipers and clockwise on Knorr-Bremse callipers with each brake application. The most movement should be on first application and then become progressively smaller.
- Replace the calliper if there are any issues such as the adjuster does not move, turns only on first brake application or goes backwards and forwards with every application.
CAUTION: To prevent injury, keep your hands away from calliper and spanner during movement.
- Calliper Adjustment. Use a spanner to set the brake calliper clearance.
- WABCO callipers have one central tappet. Adjust brake pad clearance to 1 mm (0.040”) using a long feeler gauge in the centre of the pad, between the outboard side of the calliper and the back of the brake pad. After completing adjustment, remove feeler gauge and rotate wheel hub to ensure it spins freely without any brake drag.
- The Knorr-Bremse SN7 calliper adjuster includes a shear adapter. The shear adjuster is designed to break off if excessive torque is applied. This can happen if the adjuster sticks because it hasn’t been operated for a while but can be also be from a severe problem within the calliper. It is usually best to rock the adjuster back and forth to free it up before trying to move it. If the shear adapter breaks, try again with a second shear adapter, but if this one also breaks from excessive torque then you must replace the calliper.
Knorr-Bremse brake callipers have two tappets, and therefore brake clearance must be set with two feeler gauges. Long 1.0 mm (0.040”) feeler gauges should be inserted between the tappets and the brake pad backing plate. After completing adjustment, remove feeler gauges and rotate wheel hub to ensure it spins freely without any brake drag.
- Install adjuster plug with a smear of white lithium-based grease to aid installation and to help seal against water entry.
Restore trailer to normal operating conditions and confirm proper brake function.