Hydraulic Brake Boost (HBB) is an enhanced function supported by the Electronic Stability Control Unit (ESC) in a vehicle's braking system with conventional vacuum brake booster and master cylinder. The purpose of HBB is to amplify the driver's brake pedal effort using the ESC pump where vacuum boost is no longer sufficient to help deliver the necessary support to achieve the driver's intended brake force, ie when vacuum booster run-out is achieved.

Typical cases where vacuum booster run-out could occur:

• New Direct Injection Engine Generations
• Gross Vehicle Weight Threshold Loading
• High Altitude Driving
• High Deceleration Braking
• Vacuum Booster Failure
• Engine Cold Start