Abstract: An apparatus is for sensing the presure in a vehicle's tires while the tire is revolving. Tire pressure is tapped at the valve stem. Pressurized air is passed through a delivery tube from the valve stem to a pressure sensor which detects either a too high or a too low pressure from the tire. The delivery tube passes from the valve stem over the outer face of the tire to the center of the wheel. There it enters a hole bored along the axis of rotation of the wheel and passes through the wheel and suspension parts until it reaches a free space. Here the rotating tube connects to an inner, rotary master cylinder of the sensor. The master cylinder includes a piston acted upon by the pressurized air from the tube. A spring opposes the air pressure, so that the axial displacement of the piston is proportional to the tire pressure.