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

摘要： 制动主缸作为制动系统的重要执行部件，依靠皮碗实现油液密封，产生制动压力。EPDM （三元乙丙橡胶）材质皮碗经制动液长期浸泡，会逐渐发生膨胀变形。通过浸泡耐久测试， 明确了皮碗膨胀的三个相关因素：自身材质成分、使用温度和制动液中矿物油（塑化剂）含 量。最后通过整车测试，验证了皮碗膨胀率超过 5%后，密封唇部会出现褶皱现象。这些褶皱 凹陷与缸体产生缝隙造成内部泄压，最终导致制动失效。

关键词: 制动主缸；皮碗膨胀；膨胀因素；制动失效