Four-circuit protection valve.

The four-circuit protection valve (hereinafter referred to as the four-protection valve) is composed of four pressure-limiting check valves arranged in a certain relationship, and some other functional mechanisms are appropriately added to realize the bypass supply of the air source in the commercial vehicle brake system. The circuits work independently and normally. Normally, ports 21 and 22 of each outlet of the four-circuit protection valve are opened all the way first to ensure that the vehicle has the initial braking capability to ensure driving safety, and the remaining ports are opened randomly.