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Surge Braking System

Mobilexhibit trailers are equipped with hydraulic brakes on the two rear-most wheels. The main difference between automotive hydraulic brakes and hydraulic trailer brakes are the actuation systems which transfer the braking signal from the tow vehicle to the brakes.

Brake Actuator
Manufactured by Atwood Industries, Inc.
The surge braking system uses a specially designed trailer hitch coupler, called an Actuator, which has a hydraulic cylinder built in. When the tow vehicle applies its brakes, the tow vehicle decelerates causing the trailer to apply a pushing force against the hitch. This force actuates the surge hitch hydraulic master cylinder, transferring high pressure brake fluid to the wheel cylinders (figure 1). The trailer brakes are now applied.

Figure 1: Hydraulic Brake Actuator

Uni-servo Brakes
This type of hydraulic brake uses a single acting cylinder. Upon actuation, the primary shoe is pressed against the brake drum, which causes the shoe to move in the direction of rotation. This movement in turn actuates the secondary shoe through the adjuster link assembly.


Brake Maintenance
The surge braking system on your Mobilexhibit trailer should not require any special attention with the exception of routine maintenance such as shoe and lining replacement. If problems occur, the entire tow vehicle/trailer braking system should be traced by a qualified mechanic using a methodical approach to determine the exact source of the problem.

Wheel Cylinders
Inspect for leaks and smooth operation. Clean with brake cleaner and flush with fresh DOT 3 automotive brake fluid. Hone or replace as necessary.

Brake Lines
Check for cracks, kinks, or blockage. Flush with fresh brake fluid. Bleed system to remove all air. Replace as necessary.

Shoes and Linings
Inspect visually. Replace if the lining is worn (to within 1/16" or less), contaminated with grease or brake fluid, or abnormally scored or gouged.

Check all hardware. Check shoe return spring, hold down springs, and adjuster springs for stretch or wear. Replace as required. Service kits are available.

Check drums for scoring, cracking, or uneven wear. Turn drum smooth only if under maximum diameter. Replace as necessary.

Brake Fluid
Check brake fluid level in the master cylinder reservoir. Keep filled to within ½" from top. Do not fill reservoir beyond ½" from top. Do not overfill, brake fluid will damage paint.


Brake Adjustment
Adjustment is critical to proper brake performance. Adjustment intervals are after the first 200 miles, and every 3000 miles thereafter.

  1. Raise one set of trailer wheels at a time.
  2. Remove dust clip from adjusting slot at lower part of back side of brake assembly (rear wheels only) and insert brake adjusting tool (Figure 2).
  3. Figure 2: Brake Adjustment
  4. Adjust brake shoes out by moving end of adjusting tool as illustrated, only until wheel will not turn. When this condition is felt by rotating wheel, back-off (loosen) adjustment 5 (five) notches. Check to see that wheel rotates freely.
  5. If wheel still does not rotate freely, apply pressure to brakes to expand shoes against drum and center shoes. Release pressure and check to see if wheel rotates freely. If not, repeat procedure from above.
  6. Check that all hydraulic fittings are secure. Remove master cylinder filler cap and fill the reservoir with DOT 3 automotive brake fluid.
  7. At brake assembly, connect a bleeder hose to bleeder fitting on wheel cylinder and submerge free end in a container with brake fluid. DO NOT REUSE BRAKE FLUID.
  8. Loosen bleeder fitting at top of brake assembly one turn.
  9. Apply actuator (see Figure 3) and then tighten bleeder fitting. Return actuator to released position. Again, loosen bleeder valve one turn and apply actuator.
  10. Figure 3: Bleeder Bar
  11. Repeat this procedure until the fluid expelled from the bleeder hose is free of air bubbles. During this procedure, the master cylinder reservoir fluid level must be maintained at no less than ½ full and no more than ½" from top of reservoir. Note: Use power bleeder or bar with 2" diameter hitch ball attached (Figure 3). Do not use breakaway cable for purpose of bleeding brake system.
  12. When no air bubbles are visible, close bleeder valve securely and remove the bleeder hose.
  13. Repeat steps 5, 6, 7, 8 and 9 for remaining brake.
  14. After bleeding has been completed, re-check fluid level in master cylinder.
CAUTION! CAUTION: DO NOT use brake fluid drained from brake system in refilling master cylinder as such fluid may have contaminates from the system. Use only new DOT 3 automotive brake fluid.

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