4787990031 | Genuine Wabco® Relay Valve W/ATC Kit

1. Faster Brake Response

• On long vehicles, air has to travel from the brake pedal to rear brakes.

• A relay valve receives the signal at the rear axles and applies air pressure locally.

• Result: Rear brakes engage almost instantly, reducing stopping distance and brake lag.

2. Reduce Brake System Stress

• Without a relay valve, the long air lines from the front to rear brakes could cause uneven pressure.

• The relay valve balances pressure, so rear brakes don’t get over-pressurized.

• This prevents wear and tear on brake components.

3. Traction Control (ATC)

• The ATC feature monitors wheel spin during acceleration.

• If a wheel starts slipping (like on ice or wet roads), the ATC uses the relay valve to modulate air pressure to that wheel.

• Result: Better grip, fewer accidents, more stable handling.

4. Integrated Safety and Efficiency

• Combines quick braking and traction control in one system.

• Helps drivers maintain control in slippery conditions while reducing brake wear.

1. Slow or Delayed Brake Response

• The relay valve helps rear brakes engage quickly.

• Signs of a problem:

  • Rear brakes lag behind front brakes.

  • You feel “spongy” or delayed braking when you press the pedal.

2. Uneven or Harsh Braking

• A failing valve may cause some brakes to apply too hard or too lightly.

• Symptoms:

  • The vehicle pulls to one side during braking.

  • Sudden jerks when stopping.

3. Wheel Lock or Slip During Traction Control

• ATC controls air pressure to prevent wheel spin.

• If it fails:

  • Wheels may spin excessively on slippery surfaces.

  • Vehicle may lose traction unexpectedly.

4. Air Leaks

• Relay valves have seals and diaphragms that can wear out.

• Signs:

  • Hissing sounds near the valve.

  • Air pressure dropping in the brake system.

  • Brake pedal feels hard to pump or doesn’t hold pressure.

5. Warning Lights / Error Codes

• On modern trucks with ATC:

  • ATC or ABS warning lights may illuminate.

  • Diagnostic codes may indicate valve malfunction or air pressure issues.

6. Slow Trailer Brake Response (if on a trailer)

• On vehicles towing trailers:

  • Rear or trailer brakes respond sluggishly.

  • Relay valve may not be relaying air correctly.