While certainly not new, air disc brakes are a quickly growing portion of the heavy vehicle braking systems, this is a disc brake inspection guide covering the basic principles.
Compared to drum brakes, disc brakes are not as durable in off-road or dirty environments and require more frequent maintenance inspections in these applications to detect contamination or damage before it causes severe problems.
The following inspection items are general in nature. It is important to always refer to the original equipment manufacturer’s specifications as well as this disc brake inspection guide.
Both the inner and outer brakes pads should be individually checked for wear at the top and bottom. The minimum thickness of the friction material is 2 mm. Therefore, consider the trailer service schedule and usage to ensure that there will still be adequate friction material until the next planned service check.
Compare the thickness of the friction material between the top and bottom of the same pad. The difference should be less than 2 mm. If not, replace the brake pads and inspect the guide pins and calliper installation. Excessive and tapered brake pad wear may occur if the callipers are incorrectly positioned, left to right.
WABCO PAN 19 and PAN 22 callipers have direction arrows that point in the direction of wheel rotation. If the arrows become hard to see, note that wheel rotation is from the long guide to the short guide. Hendrickson MAXX22T callipers have permanent cast arrows and are designed to prevent incorrect installation.
Compare the thickness of the inboard and outboard brake pad friction material. There must be less than 3.5 mm difference in thickness. If there is a difference in pad thickness then check for anything that could restrict calliper movement, such as seized or sticky guide pins, a tight air line or stretched wiring to the pad wear sensors. Replace pads as part of the repair.
Check the brake pads for damage or fragmentation. Minor damage at the edges is acceptable but replace the pads if major friction material damage is found.
Check brake pad spring clips for condition and tension. Failed clips can allow pads to wear unevenly and dislodge.
Visually inspect rotor for heat discolouration, contamination, scoring, runout and large or continuous cracks. Mark rotor and turn through one complete rotation to inspect the entire the braking surface.
Measure rotor thickness at 3 points, 120 degrees apart to ensure it is wearing evenly. Refer to the minimum thickness specification, which will be stamped into the edge of the rotor.
Calliper and Guides
Carefully examine the guide pin caps. Ingress of moisture, through even slightly damaged end caps, will cause the pins to seize.
Inspect adjuster guide pin boots and piston boot for cracks, tears and correct fitment.
Check for lateral movement of the brake calliper. A small amount of movement, less than 2 mm, in the inboard/outboard direction indicates that the brake calliper is sliding properly on its guide pins. If the calliper has no movement or appears to move more than 2 mm, a closer inspection will be necessary.