- Not an original spare part - Protects the surge brake's sliding rail from dust, dirt, and moisture A coupling dust cap for a surge brake protects the brake's sliding rod from dust, dirt, and moisture. During braking, the sliding rod moves inward when the trailer or vehicle pushes against it, activating the brakes. The sliding rod is made of steel and lubricated to ensure smooth operation. The coupling dust cap prevents dirt and debris from contaminating the lubricant, allowing the sliding rod to function effectively. Since the coupling dust cap is exposed to all weather conditions, it should be replaced in a timely manner to keep the surge brake well protected. Replacing the coupling dust cap: When replacing, note the type and/or brand of the surge brake (found on the label attached to the brake unit), as this determines the correct dust cap size. Alternatively, measure the diameter on the coupling side and the diameter on the surge brake side. Take note of cutouts for mounting bolts, such as those used for ball couplings. Use a 19 mm wrench to remove the bolts/nuts on the coupling and take off the old dust cap. Install the new dust cap and secure it with a tie-wrap if necessary. Reattach the coupling and tighten the bolts/nuts using a wrench or socket tool sized at 19 mm.