Patent ID: 8240780

Claim:
A hydraulically boosted vehicle brake system, comprising: a plurality of individual wheel brake actuators; a master cylinder for supplying pressure fluid to at least an individual actuator of said plurality of individual wheel brake actuators; an input member responsive to an operator brake pedal operational input force for controlling the supply of pressure fluid to the actuators; and a power boost unit for selectively supplying boost pressure to the master cylinder, the power boost unit including a housing, a generally cylindrical bore within the housing, a boost piston reciprocably located within the bore and defining therewith a variable volume boost pressure chamber, a fluid pressure source, a valve assembly for selectively supplying fluid pressure from the source to the boost pressure chamber and venting fluid pressure from the boost pressure chamber, the input member extending into the housing and coupling to the boost piston, a sensor arrangement for detecting variations in the coupling between the input member and the boost piston, and an electronic control unit responsive to the sensor arrangement for controlling the valve assembly, wherein the input member and the boost piston are coupled to each other by a lost motion coupling configured to allow-the input member to move axially in relation to the boost piston from between a brake released position and a brake applied position without causing the boost piston to move, wherein the boost piston defines a chamber, the chamber being isolated from the boost pressure chamber by a seal, wherein the input member includes a front end portion that is reciprocably retained within the chamber by the boost piston, wherein the front end portion of the input member includes a front face that faces toward the boost piston and a rear face that faces away from the boost piston, wherein the sensor arrangement includes a first sensor positioned in the chamber to be contacted by the front face of the front end portion of the input member when the input member is at the brake applied position, and wherein the sensor arrangement includes a second sensor positioned in the chamber to be contacted by the rear face of the front end portion of the input member when the input member is at the brake released position.