The Wabco HPB master cylinder kit is engineered for specific International (Navistar) and Workhorse chassis that utilize a high-pressure hydraulic pump instead of a traditional brake booster.

• International 4000 Series (DuraStar): Fits 4300 and 4400 models (typically years 2004–2014) equipped with the Wabco HPB unit.

• International School Buses: Fits IC Bus CE Series (2005–2015) that use hydraulic power braking.

• Workhorse Chassis: Commonly used on W20, W22, and W24 motorhome chassis (years 2004–2012) featuring Wabco braking systems.

• Freightliner Custom Chassis: Used on certain specialized walk-in vans and step vans.

• High-Pressure Seals: Special chemical-resistant rubber seals designed to withstand the constant high pressure of the HPB pump.

• Precision Pistons: The primary and secondary pistons that move brake fluid into the ABS modulator.

• Check Valves: Integrated valves that help maintain "standby" pressure in the lines.

• O-Rings: Specific rings to seal the master cylinder to the aluminum hydraulic power unit (HPU).

• Continuous Pump Running: The most common sign. If you hear the electric pump motor (located under the cab) running for long periods or cycling on and off every few seconds without you touching the brakes, the internal seals are likely leaking, preventing the system from holding pressure.

• "Brake Pressure Low" Dash Warning: The system has internal pressure sensors. If the master cylinder seals are failing, the system cannot maintain the required 1500–2000 PSI, triggering a warning light and alarm.

• Rapid Fluid Displacement/Aeration: If the internal seals in the kit are bad, brake fluid may "bypass" internally, causing turbulence in the reservoir. You might see tiny air bubbles or foamy fluid in the plastic tank.

• Yellow or Red ABS/Brake Lights: Since the master cylinder is integrated with the ABS modulator, a mechanical failure in the seals often throws an electronic fault code (DTC) related to "Pressure Management."

• Pedal Kickback or Vibration: During braking, if the seals are leaking, you may feel a strange pulsation or "pushing back" sensation in the pedal as the pump tries to compensate for the pressure loss.

• External Leaks at the HPU Block: If you see fluid dripping from the seam where the master cylinder meets the aluminum Hydraulic Power Unit (HPU), the O-rings or rear seals in the kit have failed.