Abstract: The compact, integrated, multi-function valve employed in semi-trailer brake systems having a plurality of pressurized air reservoirs comprise a plurality of internal passageways and ports communicating externally, a plurality of internal control means for interrupting or establishing fluid communication between the various internal passageways and a plurality of one-way means to ensure the integrity of the brake system in the event of leakage problems with one or the other of the pressurized air reservoirs. A preferred embodiment of the brake valve employs four sets of internal passageways and ports, two control means and two multi-function one-way means. As compared with the prior art, the improved brake valve protects the integrity of the brake system with less complexity, fewer components and the potential for enhanced reliability, reduced maintenance and overall lower costs.