The present invention relates generally to tandem master cylinders, and in particular to tandem master cylinders with center-compensation valve members.

Tandem master cylinders with center-compensation valve members have been provided previously such as in Schaefer U. S. Patent No. 4,550,567. A center-compensation valve member may be a part of a caging mechanism for the respective spring. It is desirable to provide a caging mechanism for the primary spring and which does not affect the center-compensation valve mechanism of the primary piston. The caging mechanism should be adjustable so that the at-rest position of the second piston may be accurately determined and thus minimize the tolerances involved in effecting the closing of the center-compensation valve member of the second piston. Additionally, the caging mechanism for the primary spring should ensure that the first and second pistons travel simultaneously while the center- compensation valves are closing during actuation.

The present invention provides solutions to the above problems by providing 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 firs piston having a pair of longitudinal extensions definin diametrically opposed circumferential slots communicating wit 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 pair of radially extending tang members extending therefrom, the tang members received within the respective slots and th radially extending end received within the central opening suc that the end part is non-rotatably coupled with the head par of the first piston, radially extendable resilient mean located within the central opening and located between th radially extending part and the shoulders of the longitudina extensions to capture said end part within the central opening, the end part including a longitudinal opening receiving therei a longitudinally adjustable member having a radially enlarge member located exteriorly of the longitudinal opening, and cage end member seated at a rear portion of said second pisto and including an end opening communicating with an interio opening such that the radially enlarged member extends throug the end opening and is received within the interior openin thereof, the radially enlarged member engaging said cage en member, the first spring being captured between the cage en member and the head part of the first piston, such tha longitudinal adjustment of the longitudinally adjustable membe relative to the end part adjusts the extension of said firs spring and determines the at-rest position of the secon piston.

One way ^\' of carrying out the invention is described i detail below with reference to the drawings which illustrat an embodiment in which:

Figure 1 is a section view of the tandem maste cylinder of the present invention;

Figure 2 is an enlarged section view of the encircle portion of Figure 1; and

Figure 3 is a partial end view taken along view lin 3-3 of Figure 2.

The master cylinder of the present invention i designated generally by reference numeral 10 in Figure l.

Master cylinder 10 comprises body 12 having extended longitudinal or axial bore 14 which communicates fluidly with reservoir 16 by not shown compensation openings. Located within bore 14 is first piston 20 and second piston 30 having diametrically extending slot openings 22, 32 located at central axial portions. The pistons 20, 30 include head parts 24, 34 supporting lip seals 25, 35, 35A, and through openings 26, 36 receiving therein spring biased valve members 27, 37. Second piston 30 includes spring 51 biasing valve member 37 toward stop pin 53, while valve member 27 of first piston 20 is biased by spring 52 toward stop pin 54. First spring 29 is captured by a caging mechanism referenced generally by numeral 60, and second spring 39 is located between end 15 of bore 14 and shoulder 38 of head 34 of second piston 30. In Figure 2, piston head 24 of first piston 20 includes a pair of longitudinal extensions 40 having therebetween a pair of oppositely disposed radial slots 42 communicating with central opening 44. End part 47 of caging mechanism 60 comprises longitudinal part 47A extending into radially extending end 47B located within central opening 44. Spring 52 biasing center-compensation valve member 27 seats on star washer 53 seated in groove 56 of extensions 40. End 47B includes a pair of oppositely extending tang members 49 which are received within slots 42 so that end part 47 is located non-rotatably relative to first piston head part 24. Radially extendable resilient means or flexible C-ring 55 is received within central opening 44 such that it is located in groove 21 against shoulders 21A of longitudinal ex ^\' tensions 40. End part 47 includes self tapping threaded end 47C received within longitudinally adjustable member 65. Member 65 includes radially enlarged member or head 66 and member opening 67 receiving the threads of end 47C. Caging mechanism 60 further includes cage end member 70 which is generally cup-shaped to define with second piston 30 an interior opening 71 communicating with end opening 72 receiving therein member 65. Cage end member 70 includes radially opposite lip openings 73 for fluid flow therethrough, and a flared flange or lip 74 providing a seat for one end of first spring 29 (see Figure l) . The opposite end of first spring 29 seats at first piston

diametral extension 28. Thus, caging mechanism 60 comprises end part 47 with radially extending end 47B coupled non- rotatably with axial longitudinal extensions 40 of first pisto 20, and adjustable member 65 with its head 66 engaging cage en member 70 which is received at the back portion 31 of secon piston 30.

Caging mechanism 60 is assembled by inserting end 47 within central opening 44 such that tang members 49 are received within the respective radial slots 42, and the inserting resilient means 55 through the smaller diameter en opening 24A of the first piston until it expands diametrically outwardly and is received within groove 21. First spring 29 is placed over head part 24 of first piston 20 until one en seats at diametral extension 28, and then cage end member 70 with member 65 is located at the other end of first spring 29 and member 65 screwed onto threaded end 47C. The member 65 is advanced along threaded end 47C until a predetermined sprin load and length for first spring 29 is attained. The first piston with caging mechanism 60 engages second piston 30 an they are inserted into bore 14 of master cylinder body 12.

When the master cylinder is actuated for a braking application by a not shown rod effecting displacement of first piston 20 toward the left in Figure 1, first spring 29 effects a simultaneous movement of second piston 30 and as piston heads 24, 34 move away from respective pins 54, 53, respective valve members 27, 37 seat sealingly so that fluid no longer flows through respective openings 26, 36. When the brakin application is completed, and the pistons return toward thei respective at-rest positions, the ends of valve members 27, 37 engage respective stop pins 54, 53 to cause valve members 27, 37 to move away from their respective seats in head parts 24, 34 and allow fluid flow through openings 26, 36. The cagin mechanism for the primary spring enables the first spring t be set at a predetermined preload and a predetermined lengt which minimizes the tolerances involved in the closing of valv member 37 of second piston 30. The tolerances are minimize by adjusting member 65 such that the resulting at-rest positio of the second piston is accurately controlled.