Brake master cylinder

Abstract

To provide a brake master cylinder that can suppress curling and cracking of a cup member.SOLUTION: A brake master cylinder 1 includes: a cylinder 11 having fluid chambers 14, 15 formed therein; pistons 12, 13 that can move back and forth within the cylinder 11 in response to operation of a brake pedal 2; and cup members 18, 19, 22, 23 that seal between the pistons 12, 13 and the cylinder 11 and that can slide on an inner circumferential surface of the cylinder 11 as the pistons 12, 13 move back and forth. At least sliding surfaces of the cup members 18, 19, 22, 23 that slide on the inner circumferential surface of the cylinder 11 are formed of a resin material such that frictional force received by the cup members 18, 19, 22, 23 and repulsive force of the cup members 18, 19, 22, 23 satisfy the relationship (repulsive force / frictional force)>1. Surface roughness Rz of the inner circumferential surface of the cylinder 11 is 2.0 μm or less.SELECTED DRAWING: Figure 1

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