Patent ID: 7147006

Claim:
A valve assembly having a fluid switch and a main valve for controlling fluid pressure in a working volume in three control states wherein the main valve has a control chamber, a moveable valve member, and three ports for fluid, namely a first port being for connection to a source of pressurised fluid, a second port being for connection to volume of low pressure fluid, and a third port for connection to the working volume, the valve member being moveable between a build position in which the second port is substantially closed and the first port is connected to the third port, a hold position in which flow of fluid through the first, second, and third ports is substantially prevented, and an exhaust position in which flow of fluid through the first port is substantially prevented whilst the second port is connected to the third port, flow of fluid through the first port and the second port being controlled by the fluid switch being provided with an electrical signal in a first state or a second state respectively, and flow of fluid through the third port being controlled by alternating said signal between said first and second states, wherein the pressure supply to the fluid switch is drawn from the source of pressurized fluid thereby making the flow into the control chamber partly proportional to the supply pressure, and the valve assembly is configured such that if the fluid pressure in the control chamber exceeds a predetermined percentage at the first port, the valve member adopts the exhaust position.