关键词: 制动主缸；皮碗膨胀；膨胀因素；制动失效

Abstract: Being the important element of execution, the brake master cylinder achieves oil seal relying on seal cup, and produce brake pressure. EPDM seal cup will keep expanding if dipping in brake fluid for a long time. Upon soaking testing of different duration, through the immersion durability test, three factors related to the expansion of the leather cup have been clarified: its own material composition, use temperature and the content of mineral oil (plasticizer) in the brake fluid. Finally, according to the testing result of whole vehicle, it comes the conclusion, the sealing lip will have fold if seal cup expands more than 5%, which make the cylinders have crevice, and resulting in internal pressure relief. Eventually it makes the braking inactive.

Key words: Brake master cylinder; Expanding of seal cup; Reasons of expanding; Brake failure