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Why is there air in the brake system again a few days after bleeding?

2 Answers

AdrianaLee

07/29/25 10:19pm

If air cannot be completely bled from the brake system, air ingress primarily occurs during brake release. When the master cylinder retracts but the wheel cylinder fails to retract, a certain vacuum is created in the pipeline. If the rubber seal of the wheel cylinder is not tight (though it may not necessarily leak brake fluid), air can gradually enter. Over time, this accumulation leads to a spongy brake pedal feel. Relevant information is as follows: 1. Brake Fluid: Brake fluid is incompressible, while air is compressible. Therefore, if there is air in the brake system's hydraulic circuit, it will affect the transmission of braking force, resulting in weakened braking performance. The bleeding process involves opening the bleeder valves at the end of each wheel's brake line, but this operation is relatively complex and should be performed by a professional repair shop. 2. Additional Information: Start the vehicle and press the brake pedal firmly. If the pedal sinks to the floor, this indicates a problem. Additionally, after starting the vehicle, if the first press of the brake pedal feels unusually low but returns to normal after a few pumps, and if this phenomenon repeats after releasing the pedal for a few minutes and pressing it again, it suggests the presence of air inside the brake system's pipelines. The most noticeable symptom is a soft or ineffective brake pedal.

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BarrettFitz

08/15/25 10:51pm

After running my auto repair shop for so many years, I've encountered quite a few similar cases. When brake systems develop problems again just days after bleeding, it's usually caused by slow leaks in the system. Common leak points include worn seals in the master cylinder, aging wheel cylinder (slave cylinder) cups, or loose/corroded brake line connections. These minor leaks allow air to seep in, making the brake pedal feel spongy and reducing braking effectiveness. My advice is don't just stop at bleeding the system - use a pressure tester to locate the leaks and make proper repairs. Otherwise, prolonged neglect could lead to brake failure at highway speeds, which is extremely dangerous. Regular checks of brake fluid levels and replacing aged brake hoses (recommended every 5 years) are effective preventive measures. Always test pedal feel with light taps before driving, and address any issues immediately. Safety should never be compromised to save a few bucks.

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