When using the pump, connect its air outlet pipe to the valve of the bicycle tire. The function of the valve is to allow only air to enter the tire from the pump, and not allow air to flow from the tire into the pump. The piston and the wall of the pump There is a gap between the pistons, and there is a concave rubber bowl on the piston. When the piston is pulled up, the volume of air under the piston increases, the pressure decreases, and the air above the piston squeezes from the periphery of the rubber bowl to the lower side. When the air volume under the piston is reduced, the pressure is increased, so that the rubber bowl is pressed against the wall of the tube to prevent air from leaking over the piston, and the piston is continuously pressed downward. When the air pressure is sufficient to open the valve core of the tire, the compressed air is compressed. Enter the tire. At the same time, the air outside the cylinder enters the piston from the gap at the upper end of the cylinder.