A pump includes a base and a cylinder installed on the base. The base defines a pumping channel. The cylinder defines a first chamber and a second chamber with a diameter different from that of the first chamber. The first and second chambers are in communication with the pumping channel. A first check valve only allows air to travel into the pumping channel from the first chamber. A second check valve allows air to travel into the pumping channel from the second chamber. A first piston is movable in the first chamber. A second piston is movable in the second chamber. A control device prevents the venting of air from the chambers in a first mode, vents air from the second chamber without traveling through the pumping channel in a second mode, and vents air from the first chamber without traveling through the pumping channel in a third mode.