Why Shouldn't the Brake Fluid Cap Be Over-tightened?
2 Answers
For more information on brake fluid, please refer to the following: 1. Brake fluid, also known as hydraulic brake fluid, is the liquid that transmits force in the braking system. It is a synthetic oil composed of various additives and has strong hygroscopic properties. 2. According to the American Petroleum Institute standards, brake fluids are classified into three types: DOT3, DOT4, and DOT5.1. DOT3 is typically an alcohol-ether type, while DOT4 and DOT5 are ester-based and silicone-based, respectively. 3. Regular maintenance of brake fluid is essential. Generally, the maintenance cycle for brake fluid is time-based, with a replacement interval of two years. Some manufacturers recommend two years or 40,000 kilometers, whichever comes first. Specific details can be found in the maintenance manual. 4. Whether brake fluid needs replacement mainly depends on its water content. Due to its high hygroscopicity, over time, a significant amount of air bubbles can form in the brake lines. These bubbles can severely impair the performance of the braking system. When the brake system operates, it generates high temperatures, causing the water in the brake fluid to vaporize. This makes the gas easily compressible, leading to dangerous phenomena such as soft brakes and reduced braking power. During each maintenance session, a brake fluid tester should be used to assess the condition of the brake fluid.
I've been driving for over twenty years and found that overtightening the brake fluid cap is a big issue. I remember once during a repair, I tightened it hard, and the cap got stuck so firmly that even a wrench couldn't loosen it. In the end, I had to remove the entire reservoir and replace it with a new part. The brake system has vent holes to balance pressure, and if the cap is too tight and blocks them, it can cause a vacuum effect, affecting brake performance. Especially when going downhill or during sudden braking, the fluid pressure changes significantly. If the cap isn't breathable, brake response can slow down. For safety, hand-tightening until you feel slight resistance is enough—no need to go further. I've also heard of cases where overtightening damaged the seal ring, causing fluid to leak and contaminate other parts. When checking the fluid level regularly, just lightly tighten the cap. Learning these small tips can prevent major troubles and ensure driving safety. Reading fellow drivers' shared experiences also broadens knowledge.
