A master cylinder (10) having a sleeve (24) for positioning first (22) and second (30) seal members within a bore (14) of the housing (12). The first (22) and second (30) members each have a primary seal (52), a retainer (58) and secondary seals (54 or 55). The primary seals (52) have a base member (60), a peripheral ring (64) and a flexible section (66). The retainers (56) have a groove (70) for holding the base member (60), radial slots (74, 74') connect to a source of fluid and either a rib (100) or projection (94) for positioning the secondary seals (54 or 55). First (38) and second (40) pistons which engage the first (30) and second (22) seal members, have communication passages (80, 82) through which the radial slots (74, 74') (75, 75') are connected to first (48) and second (42) chamber. In response to an input force, the first (38) and second (40) pistons move into the first (38) and second (42) chambers, respectively, to pressurize fluid therein after interrupting communication through the passages (74, 74') (75, 75'). The fluid pressure acts on the base members (60, 60') to seal the chambers (46, 42) from the source of fluid (23). As the first (38) and second (40) pistons move out of the chambers (46,42), a vacuum may be created therein. The flexible sections (66, 66') allow the peripheral rings (64, 64') to move and permit the vacuum to draw fluid into the chambers (46, 42) to assure that a substantially constant volume of fluid is available to effect a brake application.