A device for Controlling pressure is disclosed which comprises a pump (28) having opposed pistons (30) which reciprocate in cylinders (40) carried by a casing (38). The pistons (30) are at the outer ends of rings (50) which are themselves fitted to an eccentric (26). The eccentric (28) is integral with a clutch plate (24). A second clutch plate (22) is fixed to a non-rotating axle (18) which itself forms part of a hanging counterweight structure (20). A piston (84) pushes the eccentric and clutch plate (26, 24) against the clutch plate (22) thereby to prevent the eccentric (26) tire from rotating with the rings (50) upon a sensor detecting that a tire has a low pressure in it. Stopping the eccentric initiates the pumping action of the pistons (30).