Patent ID: 7895837

Claim:
A differential hydraulic master cylinder having a cylinder body in which is provided a first working chamber and a second working chamber, the second working chamber having a smaller diameter than the first, the cylinder further having a first fluid passageway for fluidly connecting the first chamber with a source of ambient pressure and a pressure relief valve in the first passageway which is arranged to open to connect the first chamber with said source of ambient pressure when the pressure in the first chamber reaches a threshold value, and a second fluid passageway for fluidly connecting the first chamber with an outlet, a second valve located in the second passageway which is arranged to permit hydraulic fluid to flow from the first chamber to the outlet but to prevent hydraulic fluid from flowing in the reverse direction, at least when the pressure relief valve is open, the second valve and the pressure relief valve each comprising a movable valve member, the movable valve members of the two valves being mechanically interconnected, the second valve movable member comprising a piston located in a second valve chamber which forms part of the second fluid passageway, the piston carrying a lip seal for contact with the surface of the second valve chamber, the arrangement being such that, in use, fluid is able to flow from the first working chamber over the lip seal to the outlet but is prevented from flowing in the reverse direction by engagement of the lip seal with the surface of the second valve chamber.