In an air suspension system of a vehicle the height of the vehicle platform is maintained in use by height control valves. To facilitate loading, the height may be adjusted by an operator by overriding the height control valves by means of a selector valve. The height may then be adjusted by a Raise/Lower valve. If the operator omits to return the selector valve and/or the Raise/Lower valve to their normal condition, the selector valve is reverted to its normal condition by a pilot line connected to a service brake line so that the vehicle automatically returns to its normal ride height on the first brake application. Alternatively the valve may be operated by a solenoid connected to the brake light circuit so that the service brake line is not affected. In an alternative arrangement, the suspension system height is adjusted by two manually-operable spring-return valves. Again, a selector valve reverts to a condition in which ride height is attained on application of the service brake.