Will changing the wheel hub cause air to enter the brake system?
2 Answers
Leakage in the brake lines can cause air to enter the brake system. Braking principle of current passenger cars: Most modern passenger cars use hydraulic braking. If water accidentally enters the system during the addition or replacement of hydraulic fluid, it can vaporize under high temperatures during use and emulsify the hydraulic fluid, leading to the presence of air in the brake system. Additionally, damage or leakage in the brake lines can also cause air to enter. The presence of air in the brake system manifests as reduced braking force and a softer brake pedal. If such symptoms occur or if air is confirmed to have entered the brake system, prompt repair is necessary, as braking is crucial for driving safety. Reasons for air entering the brake lines: Air primarily enters the brake system when the brakes are released. If the master cylinder retracts but the wheel cylinder does not, a certain vacuum is created in the lines. If the wheel cylinder seal is not tight (which may not necessarily cause oil leakage), air can gradually enter over time, leading to a softer brake feel.
In my years of repairing cars, I've encountered similar situations. When replacing rims, if the brake system isn't disturbed—like only removing the lug nuts—air usually doesn't enter the brake lines. However, in practice, rim replacement often requires moving the brake caliper or disc, especially with modern cars' compact designs. Once you disconnect hoses or fittings, air can easily seep into the brake fluid. This makes the brake pedal feel spongy, drastically reduces braking efficiency, or even causes sudden failure, which is extremely dangerous while driving. I remember last year when helping a neighbor replace rims, he complained about soft brakes. Upon inspection, we found trapped air, and bleeding took half an hour. So, I recommend testing pedal firmness before and after rim replacement—pump it a few times. If it feels mushy, immediately use a bleeder tool to address it—safety always comes first. If DIY-ing, keep brake fluid and an assistant ready, and work meticulously.