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 the piston against the bias into air-tight engagement with the body to prevent passage of the air into the cylinder and, upon the occurrence thereafter of an undesirably low tire pressure, the piston is biased against this undesirably low air pressure to allow the air into the cylinder past the piston to pass by way of the perforated diaphragm and opening to inflate and rupture the sack to provide an audible indicaton of the undesirably low tire pressure.