Trailer Hydraulic Disc Brakes: Draining, Cleaning, Topping Up, and Bleeding Guide

Trailer hydraulic disc brakes are a vital component of any towing setup, ensuring smooth and reliable braking performance. However, over time, brake fluid can become contaminated or depleted, necessitating getting your hands & knees dirty to keep your brakes in tip top condition. In this guide, we will walk you through the steps to drain, clean, top up, and bleed your trailer hydraulic disc brakes, ensuring safe and efficient braking on your journeys. 

Hydraulic brake fluid should be routinely changed out every couple of years. The hydraulic fluid is hygroscopic, which means it absorbs moisture from the air and over time contaminates it to a point where it becomes more water than brake fluid.

Trailer brakes.JPG

This is especially the case in areas with high humidity and on boat trailers being used in and around water.

As well as causing internal corrosion and pitting of brake components, the water can cause major problems when heavy braking occurs. Good clean brake fluid has an extremely high boiling point and under normal braking conditions can dissipate the heat generated without issue, but when the absorbed water in contaminated brake fluid heats up, the water turns to steam and when compressed under load, can saturate the fluid with air bubbles causing loss of braking power and potentially a serious accident.

Step 1: Get the Necessary Tools and Supplies before embarking on the maintenance process, :

  1. Brake fluid (DOT 3 or DOT 4,) readily available from most car part specialists and well stocked petrol/gas stations
  2. A brake bleeding kit (including a clear plastic hose, a clean jar, and a spanner to fit the caliper bleed screw)
  3. Safety goggles and gloves
  4. A jack or jack stands (if required)
  5. Some clean rags or lint free paper towels
  6. Brake cleaner spray
  7. A mate to help out - not always necessary but handy to have

Step 2: Ensure your trailer is parked on a flat and stable surface. Engage the tow vehicle parking brake and place wheel chocks behind the wheels to prevent any unintended movement. Jack the trailer up and remove the wheels that are fitted to the braking hubs. Use axle stands to secure the trailer in position.

Step 3: Locate the hydraulic brake reservoir on your trailer. It is usually mounted near the hitch or on the trailer drawbar. Remove the reservoir cap to access the fluid.

Step 4: Draining the Brake Fluid

  1. Clean the area around the brake reservoir to prevent dirt and debris from entering.
  2. Attach the clear plastic hose to the brake bleeder valve on one of the brake calipers. Place the other end of the hose into the catch container or jar.
  3. Using the spanner, loosen the bleeder valve by turning it counterclockwise about half a turn. This will allow the old brake fluid to flow out through the hose. There is the high possibility that the bleed screw is corroded and tight, if so, spray some WD-40 or similar on the bleed screw and leave for a few minutes before trying again. Be careful not to use too much force to undo the bleed screw, they are prone to snap or round off.
  4. Gently pump the brake lever on your trailer or in the case of Electric or Air over Hydraulic systems (EOH or AOH), get your mate to turn the tow vehicle ignition on and pump the tow vehicle brakes to activate.. This will push the old fluid out through the hose. Make sure that the brake reservoir does not empty out and continuously top the fluid up during this time.
  5. Continue pumping the brake lever or pedal until the fluid runs clear, ensuring all old fluid and air bubbles are purged from the system. Again, monitor the fluid level in the reservoir to avoid running it dry.
  6. Gently tighten the bleed screw and repeat the process with the other caliper until all fluid into the catch container of jar runs clear.
  7. Again, gently tighten the bleed screw up and clean up any drips and spills. Discard the contaminated fluid appropriately.

Step 5: Cleaning the Brake Components

  1. Once the fluid is drained, clean the brake calipers, rotors, and pads using a brake cleaner spray.
  2. Remove any accumulated grime, dust, or debris from these components using a clean cloth or rag.
  3. Inspect the brake pads for wear and tear. This is a great time to replace them if necessary.

Step 6: Topping Up the Brake Fluid

  1. With the brake system drained and cleaned, refill the reservoir with fresh brake fluid,
  2. Slowly pour the fluid into the reservoir, avoiding spills or overfilling.
  3. Pay attention to the recommended fluid level indicated on the reservoir. Do not exceed the maximum fill line.

Step 7: Bleeding the Brake System

  1. Starting with the brake caliper farthest from the reservoir, attach the clear plastic hose to the bleeder valve and place the other end into the catch container or jar.
  2. Get your mate or use a brake bleeding kit with a one-way valve to prevent air from being drawn back into the system.
  3. Open the bleeder valve slightly, and have your mate slowly depress the brake pedal or activate the brake lever.
  4. As your mate holds the pedal or lever down, tighten the bleeder valve.
  5. Instruct your mate to release the pedal or lever, allowing it to return to its original position. Repeat this process until no air bubbles are visible in the clear hose.
  6. Move to the next caliper and repeat the bleeding process, working your way from the farthest to the nearest caliper.

Step 8: Final Checks and Testing

  1. Once all calipers are bled, ensure the brake fluid level is correct in the reservoir.
  2. Check for any leaks around the bleeder valves or the brake system components.
  3. Reinstall the reservoir cap securely.
  4. Test the brakes by applying gentle pressure to the brake pedal or lever. Ensure they engage smoothly and provide adequate stopping power.
  5. Take a short test drive to check the proper operation of the trailer's hydraulic disc brakes.

Maintaining the hydraulic disc brakes on your trailer is essential for safe towing. Regular draining, cleaning, topping up, and bleeding your trailer's hydraulic disc brakes, will ensure their optimal performance and reliability. Remember, if you encounter any difficulties or are unsure about the process, get some advice from a qualified mechanic.