What Causes Sudden Brake Softness Followed by Normal Recovery?
3 Answers
Sudden brake softness followed by normal recovery occurs because the brake booster pump only starts working after the car is started. During normal driving, the vacuum booster pump assists the braking system, but it stops functioning when the engine is turned off, leaving no vacuum force inside the brake system. Brakes can be categorized into linear and non-linear types: Brakes are divided into linear and non-linear types. The so-called "soft brake" refers to non-linear braking, meaning the force applied to the brake pedal does not proportionally translate to the actual braking effect, requiring deeper pedal depression to achieve the desired braking performance. Factors affecting brake softness or firmness: Brake softness or firmness is generally related to the vehicle's tuning. Most Japanese cars tend to have softer brakes, which is advantageous for driving on congested urban roads, as city driving rarely requires slamming the brakes to the floor.
Last time I experienced a sudden softening of the brakes, which scared me into getting it checked immediately. The mechanic said this was mostly likely due to air entering the brake system, especially if the brake fluid replacement wasn't done properly. Another possibility could be insufficient brake fluid level, slight leakage in the brake lines, or deformation due to squeezing. The most easily overlooked cause is the aging of the brake master cylinder seals, which may leak a bit when temperatures rise and then return to normal when temperatures drop. I remember after the repair, the mechanic reminded me to regularly check the brake fluid level and not wait until problems arise before taking action. Such issues absolutely cannot be delayed—it's a matter of life and death. Sudden brake failure on the highway is no joke. It's recommended that if you notice any abnormalities with your brakes, you should stop and check immediately—safety first.
With twenty years of driving experience, I've dealt with many cases of soft brakes. The common cause is brake pads worn to the limit, increasing the gap and leading to longer pedal travel. Another possibility is a stuck brake caliper piston, causing inconsistent pedal feel due to poor retraction. One detail to note: prolonged downhill braking can temporarily soften the brakes due to overheating—this is called brake fade. I recommend having technicians check brake pad thickness during every maintenance service, rather than waiting until the wear indicator makes noise. Develop the habit of anticipating road conditions to reduce sudden braking. I've experienced soft brakes twice in rainy conditions, later learning it was due to a water film on the rotors affecting friction coefficient.
