Closed height control device for vehicles

The vehicle suspension system comprises air springs (6a - 6d) fed with compressed air from a tank (12) via a compressor (8). An air drier (10) is fitted in the line (4) between the tank and compressor. A branch pipe (40) is fitted between the drier and compressor outlet (16) and links it to an outlet valve (30) which releases compressed air into the atmosphere.