Abstract: An air pressure regulator includes a housing having inlet and outlet ports extending along a common axis. A movable valve member and a piston are positioned in the housing on opposite sides of a housing flow passage. The movable valve member is adjacent the inlet port while the piston is adjacent the outlet port. The piston has a central stem extending through the housing flow passage in radially-spaced relationship thereto for engaging the movable valve member. The piston is normally biased in a direction toward the movable valve member for holding same displaced from a seat around the housing flow passage. When a predetermined pressure exists at the outlet port, the piston moves away from the movable valve member for allowing same to move into engagement with a seat around the housing flow passage for closing such passage.