- Not an 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 as the trailer or towed unit applies pressure, 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 grease, 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 maintain proper protection of the surge brake. Replacing the coupling dust cap: When replacing, note the type and/or brand of the surge brake (indicated on the label located on the brake unit), as this determines the correct size of the dust cap. Alternatively, measure the diameter on the coupler side and the diameter on the surge brake side. Note cutouts for mounting bolts, such as those used for ball couplers. Use a 19mm wrench to remove the bolts/nuts on the coupler and take off the old dust cap. Install the new coupling dust cap and secure it with a tie-wrap if necessary. Reattach the coupler and tighten the bolts/nuts using a 19mm wrench or socket.