Hydraulic master cylinders

Claim 1

A hydraulic master cylinder comprising: a reservoir for hydraulic fluid; a body defining a primary bore and a secondary bore co-axial with and having a smaller cross-sectional area than the primary bore; a piston rod slidably and sealingly mounted in the secondary bore; a piston connected to the piston rod and slidingly and sealingly mounted in the primary bore, the piston dividing the primary bore into a primary chamber located on the side of the piston remote from the piston rod and a secondary annular chamber located between the piston rod and the wall of the primary bore; an outlet for supplying fluid from the primary chamber to hydraulic apparatus; an expander chamber in fluid communication with both the primary chamber and the secondary chamber; a movable expander member located within the expander chamber and dividing the expander chamber into a primary portion which is in fluid communication with the primary chamber and a secondary portion which is in permanent fluid communication with the secondary chamber, the expander member being movable in response to a predetermined pressure differential between the primary and secondary chambers to expand the volume of the primary portion of the expander chamber to receive fluid from the primary chamber and to contract the volume of the secondary portion of the expander chamber and thereby supply fluid to the secondary chamber; means permitting fluid to flow from the reservoir into the secondary chamber until said predetermined pressure differential is reached; and means thereafter preventing fluid flow between the reservoir and the secondary chamber whereby, during brake application, after said predetermined pressure differential is attained said secondary chamber is pressurized and the effective cross-sectional area of said master cylinder is the cross-sectional area of the secondary bore.