Abstract: A low pressure warning cap for a pneumatic tire air valve including a generally cylindrical body having first and second ends, a cylinder adjacent the first end, a threaded connector for attachment to a conventional air valve stem of a pneumatic tire at the second end and an acting chamber connecting with and disposed intermediate the cylinder and the threaded connector, a piston disposed in the cylinder and resiliently biased toward the acting chamber, the cylinder having an opening at the first end, a deformable diaphragm disposed across the acting chamber between the threaded connector and the enlarged end, a perforated diaphragm disposed across the cylinder adjacent the opening, an elastic sack enveloping in air tight manner the first end including the opening, and a protuberance in the threaded connector to cooperate with the valve to open the valve upon attachment of the cap to valve, whereby, at a desired tire pressure, the deformable diaphragm is deformed by air from at the desired pressure to move th