Piston seal member and disc brake using the piston seal member

A piston seal member which holds a cylinder bore and a piston that slides in the cylinder bore in a fluid-tight and slidable manner,

• the piston seal member being formed of a crosslinked rubber composition, the rubber composition including 100 parts by weight of an ethylene-propylene rubber, 20 to 60 parts by weight of a first carbon black, 50 to 100 parts by weight of a second carbon black, and 5 to 20 parts by weight of carbon nanofiber having an average diameter of 0.5 to 500 nm, the total amount of the first carbon black and the second carbon black being 70 to 160 parts by weight,
• the first carbon black having an average particle diameter of 35 to 100 nm and a DBP absorption of 50 to 200 ml/100 g, and
• the second carbon black having an average particle diameter that is greater than the average particle diameter of the first carbon black and ranges from 60 to 500 nm and a DBP absorption of 5 to 50 ml/100 g.