Abstract: A cylinder accommodates an elongated bellows tube one end of which is connected to the cylinder and another end of which has a piston sealingly mounted in it so that the piston can move with the bellows tube during expansion and contraction of the latter. The piston in part bounds a pressure chamber in the cylinder outside the bellows tube and is urged to a position in which it closes the chamber. Channels communicate the chamber with one or more tires so that the piston is subject to the inflation pressure of the tire and shifts in the cylinder under concomitant change in the axial length of the bellows tube when changes in the tire inflation pressure occur.