A composite suspension system for a vehicle includes a mechanical suspension system supporting the body of the vehicle with respect to an axle and a pneumatic suspension system to selectively provide support for the body relative to the axle. Under heavy loads, the pneumatic suspension system may be inflated to support the body relative to the axle. When not under heavy loads the pneumatic suspension system may be deflated so as not to interfere with the support provided by the mechanical suspension system. A control system regulates a compressor connected to the pneumatic suspension system to inflate the same. The control system also regulates access of the pneumatic suspension system to a vacuum source to deflate the same.