VARIABLE CAPACITY VANE PUMP WITH DUAL CONTROL CHAMBERS

The invention relates to a method of operating a variable capacity pump at at least two selectable equilibrium pressures. The pump comprises a first control chamber, a second control chamber, a pump control ring pivotable to alter the capacity of the pump, and a return spring biasing the pump control ring towards a position of maximum volumetric capacity. The method comprises the following steps: Supplying pressurized fluid to the first control chamber to create a force on the pump control ring such that the force pivots the pump control ring to reduce the volumetric capacity of the pump, wherein the return spring acts against the pivoting force of the first control chamber to establish an equilibrium pressure; and selectively supplying pressurized fluid to the second control chamber to create a force on the pump control ring such that the force pivots the pump control ring to reduce the volumetric capacity of the pump, wherein the return spring acts against the pivoting force of the first and second control chambers to establish a new, lower, equilibrium pressure.