Within a single envelope or housing, this full-function valve integrates all of the emergency and service requirements of a tandem axle air brake system for heavy duty truck semi-trailers, thereby decreasing complexity and cost and enhancing safety, reliability and operating capability while also satisfying applicable Federal Regulations. The housing encloses a spring brake valve module, a relay valve module and a biased one-way pressure protection valve, all interconnected to meet operating requirements, while preventing non-intentional spring brake engagement, foreclosing trailer movement without adequate service brake capability and optionally providing anti-compounding protection.