What is the Housing Pressure Altitude Advance (HPAA) system?

The HPAA is a system in a diesel engine that regulates the housing pressure in accordance to the altitude. The Housing Pressure Altitude Advance system is located in the fuel return line of the engine. When HPAA solenoid is on, it has a glass check ball that is seated and regulates pressure at 5 psi. When this system is off it gets moved off its seat, and does not regulate the fuel return line.