523185 | Genuine Bendix® Tractor Protection Valve

  - Heavy-duty vehicles with air brakes
  - Semi-truck tractors (Class 7–8 trucks)
  - Sleeper trucks / highway tractors
  - Day cab trucks
  - Dump trucks (with trailer air systems)
  - Fleet and commercial hauling trucks

 1. Protects tractor air supply

• If a trailer air line bursts or disconnects, the valve automatically shuts off airflow

• This keeps enough compressed air in the tractor so its brakes still work

2. Prevents total brake failure

• Without it, air could escape through the trailer system

• That could cause complete brake loss in the tractor

3. Controls air to the trailer

• It allows air to flow to the trailer only when conditions are safe

• Works together with the trailer supply control (red knob in the cab)

4. Emergency response

• If pressure drops below a safe level, it:

  • Closes the air supply

  • Helps trigger emergency braking on the trailer

 1. Constant air leakage

• You hear hissing near the valve or trailer lines

• Air pressure drops even when everything is connected properly

  -This usually means internal seals are worn or damaged

2. Tractor air pressure won’t hold

• Air pressure in the tractor system keeps dropping

• Takes longer than normal to build pressure

  -The valve may not be sealing off the trailer circuit correctly

3. Trailer brakes not working properly

• Trailer brakes feel:

  • Weak

  • Delayed

  • Not engaging at all

    -The valve might not be allowing proper air flow to the trailer

4. Trailer supply (red knob) pops out unexpectedly

• The trailer air supply control in the cab pops out on its own

  -This happens when pressure drops below a safe level—possibly due to valve failure

5. No air going to the trailer

• After connecting, the trailer doesn’t receive air

• Brakes stay locked or don’t release

  -The valve could be stuck closed or blocked

6. Valve doesn’t close during a failure

• In case of a hose break, air continues escaping

  -This is dangerous—the valve is supposed to shut off immediately

7. Irregular brake behavior

• Sudden or uneven braking between tractor and trailer

   -Can indicate inconsistent valve operation