Center valve master cylinder with adjustable caged primary spring

A tandem master cylinder, comprising a housing having therein an axial bore, first and second pistons located within the bore and defining therewith respective first and second pressure chambers, each piston including a head part having a valve member located within a through opening of the head part, and a diametral opening, a pair of axially spaced-apart pins extending into said bore and received within the diametral openings of the respective first and second pistons, each valve member engaging a respective pin in a rest position of the respective piston so that the valve member is open and permits fluid flow through the respective through opening, a first spring extending between the head part of the first piston and a rear portion of said second piston, and a second spring extending between the head part of the second piston and end of said bore, the improvement comprising the first spring being maintained by a caging mechanism which controls the extension of the first spring such that the location of the second piston in an at-rest position is accurately controlled, the caging mechanism comprising the head part of the first piston having a pair of longitudinal extensions defining diametrically opposed circumferential slots communicating with a central opening located between the longitudinal extensions, the extensions having shoulders adjacent the central opening, an end part having a radially extending end with at least a pair of radially extending tang members extending therefrom, the tang members received within the respective slots and the radially extending end received within the central opening such that the end part is non-rotatably coupled with the head part of the first piston, radially extendable resilient means located within the central opening and located between the radially extending part and the shoulders of the longitudinal extensions to capture said end part within the central opening, the end part coupled with a longitudinally adjustable member having a radially enlarged member, and a cage end member seated at a rear portion of said second piston and including an end opening communicating with an interior opening such that the radially enlarged member extends through the end opening and is received within the interior opening thereof, the radially enlarged member engaging said cage end member, the first spring being captured between the cage end member and the head part of the first piston, such that longitudinal adjustment of the longitudinally adjustable member relative to the end part adjusts the extension of said first spring and determines the at-rest position of the second piston.