The outstanding feature is it's simplicity of operation. The front and rear Hydrolastic displacers on each side of the car are inter-connected by a small bore pipe. The system is hermetically sealed and therefore never needs any further attention during the normal working life of the car. Each displacer incorporates a rubber spring, and damping of the system is achieved by rubber valves. So when a road wheel is deflected, fluid is displaced to the corresponding suspension unit. In turn, it is raised in anticipation of its wheel encountering the cause of its counterpart's deflection. The rubber springs are only slightly brought into play and the car is freed from any tendency to pitch, although full play is given to wheel movement, producing a soft ride.

When front and rear wheels encounter a simultaneous deflection, the fluid suspension stiffens in response to the upward motion and, while acting as a damping medium, transfers the load to the rubber springs giving a controlled, vertical, but level motion to the car.

The restriction of the fluid flow, imposed by the small bore piping, rises with the speed of the car. The ride is therefore steadied at high speeds and softened at low speeds a most satisfying condition hitherto only achieved by complex and costly means.

Not to be confused with the Hydro-pneumatic suspension used by Citroen. Their system is not a hermetically sealed pressure system like the Hydrolastic system used by BMC.

Citroen use a hydraulic pump system that raises and lowers the car to different heights. A more superior system but more costly to manufacture and maintain. The extraordinary thing about their Hydro-pneumatic system, is that it doesn't use o-rings as seals. The pistons and bores are machined to incredible tolerances (microns), making seals unnecessary.

The hydrolastic suspension system contains special hydrolastic fluid, a version which is still available. I recommend using it if you're going to add to your system that you consider first completely depressurizing the system and then adding new clean fluid. The reason I recommend this, is that the car's system will be full of rust and other crud.