It can be tricky to find the cause of dragging brakes on a trailer. Several problems or malfunctioning parts can make trailer brakes activate by themselves. Dragging brakes usually don't lock up completely. It's common for the wheels to still turn, but skid as they move across less solid surfaces like gravel. Brakes that drag even slightly can dramatically lower the fuel efficiency of your vehicle. Dragging brakes can overheat and damage other parts of the brake system. Locked brakes can wear a hole in the tire as it drags on the ground. It's important to fix dragging brakes immediately.
When refilling a trailer's brake fluid, air gets into the brake lines. As the brakes are used, the air heats up and expands. The increased air pressure can keep brake shoes in contact with the brake drum as it passes through the lines. To remove air from the brake lines, you have to bleed the brakes. Most brakes have bleeder valves to assist with removing air. Hook a tube to the bleeder valve, place the other end in a container to hold the brake fluid that was released, have someone inside the vehicle press the brake pedal and open the bleeder valve slightly. Brake fluid and air will flow through the valve into the tube. The process should be repeated until air no longer comes through the valve.
Water can get into a trailer's brake system and cause corrosion. This is especially problematic with boat trailers that are exposed to saltwater. The saltwater corrodes metal parts much faster than freshwater. Corroded metal parts are not able to move as freely as normal and can become locked in position rubbing against the brake drum. It is possible to remove rust from metal parts by sanding them, but it can be difficult to remove all the rust without slightly altering the shape of the parts. It's usually a better idea to just replace corroded parts in the brake system.
If the brakes started dragging after you hit a bump or pothole, the likely cause is a bent spindle. The spindle holds a tire onto the trailer's axle. When the spindle is bent, the tire has an uneven rotation and often rubs against brake pads. You can check for a bent spindle by placing a level on top of the tire that is dragging. The level should indicate the tire is evenly balanced if the spindle is not bent. If the spindle is bent, the entire axle should be replaced. It's possible to weld a new spindle onto the current axle, but the heat required for welding weakens the metal axle and greatly increases the chance the axle will break in the future.
