Why is the car's brake pedal soft?
3 Answers
There are several reasons why a car's brakes may feel soft: Air in the brake lines: Brake fluid is the medium that transmits force in the braking system. If air enters the lines, the brake fluid under high pressure will appear insufficient in strength. Although braking force still exists, the maximum braking force is significantly reduced. In this case, the brake system should be promptly bled at a repair facility. Long overdue brake fluid change: If brake fluid is not replaced in time, it will contain excessive impurities and moisture, which will affect the transmission of braking force. Therefore, brake fluid must be changed regularly, and if possible, a moisture content test should be performed at a repair facility. Oil leakage or excessively soft brake materials: Generally, oil leakage from the master cylinder or brake lines will cause the vehicle's brakes to become soft. Additionally, if the brake materials are too soft, their high-temperature stability will deteriorate, which can also result in soft brakes.
When I used to drive a taxi, I encountered the issue of spongy brakes several times. It felt like stepping on cotton, requiring extra force to bring the car to a gradual stop. The most common cause is air trapped in the brake fluid, especially after replacing brake pads without properly bleeding the system. Additionally, aged or water-contaminated brake fluid loses its hydraulic effectiveness. Unevenly worn brake discs or leaking wheel cylinders can also lead to increased pedal travel. This problem should never be ignored—last time I neglected it, I nearly rear-ended another car at an intersection. I recommend immediately visiting a repair shop to check the fluid and bleed the system. During routine maintenance, always ask the mechanic to pay special attention to the brake fluid reservoir's level and color changes.
Yesterday while driving, I noticed something was off with the brakes. Lightly pressing the pedal barely had any effect, and I had to press it down a couple more inches before the brakes engaged. I asked my friend who works as an auto mechanic, and he said it's likely that air has entered the brake system, with air bubbles in the lines preventing the brake fluid from transmitting pressure properly. It could also be that the brake pads have worn thin and need their clearance adjusted, or that the master cylinder's seals have aged and are leaking fluid. He specifically warned me not to mix different grades of brake fluid, as doing so could cause the fluid to deteriorate. He also taught me a simple way to check: after starting the car cold, press the brake pedal several times—if the pedal slowly rises back up, it indicates a leak. Now I've made it a habit to glance at the brake fluid reservoir's level every time I wash the car.
