Why does the brake become soft again a few days after bleeding the air?

[ArthurLee]

There is still air in the brake system. Here are some explanations about why the brake becomes soft: 1. Cause: Air enters the brake system mainly when the brake is released. The master cylinder returns to its position while the wheel cylinder does not, creating a vacuum in the pipeline. If the seal of the wheel cylinder rubber ring is poor (which may not necessarily cause oil leakage), air can enter gradually. Over time, the amount of air increases, leading to the symptom of a soft brake. 2. Symptoms of air in the brake system: The feeling when pressing the brake pedal is completely different, with a sensation of gas compression and rebound. Air in the hydraulic line affects the transmission of braking force, weakening the braking power.

[DelMicah]

I helped a friend with a similar issue last time. Actually, if the brakes feel soft again after bleeding, it might be because the air wasn't fully removed or more air got in. Brake systems are very sensitive to air—if bleeding isn't done thoroughly or the master cylinder is faulty, air can flow back in. Sometimes, poor sealing, like worn-out rubber cups in the wheel cylinders, can cause air or fluid leaks. Also, brake fluid is highly hygroscopic and may absorb moisture, which can vaporize under heat and create bubbles, leading to soft brakes. My suggestion is to bleed the system again, ensuring every wheel is checked and all connections are tightened. Regular maintenance is crucial—change the brake fluid periodically, typically every two years, or it could be dangerous if the brakes fail at high speeds. If the problem persists after replacement, have a professional shop inspect the seals. Don’t skimp on this—safety comes first.

[Ribeiro]

I've been driving for over a decade and encountered this situation several times. It's usually caused by internal leaks in the braking system, such as wear on the master cylinder piston seals or rusted/leaking guide pins in the wheel cylinders. Air can get mixed in primarily due to incomplete bleeding during the last service or cracks in the brake lines allowing air ingress. I remember one instance when the brakes became soft just days after changing the fluid - turned out the reservoir cap was loose and absorbed moisture. Since then, I've made it a habit to check all connections during every maintenance. Never delay fixing soft brakes, as it increases stopping distance and could cause accidents. If doing it yourself, using specialized bleeding tools is more reliable - make sure to pump multiple times to fully remove residual air bubbles.