What Causes the Brake Pedal to Slowly Sink at Idle?
4 Answers
If the brake pedal continues to sink downward after braking, possible causes include a broken pedal spring, brake master cylinder leakage, damage to the brake master cylinder, or a faulty vacuum booster pump. Below is an analysis of several common brake failure causes: 1. Steering wheel deflection during braking: If the steering wheel turns to one side when the brake is applied, this indicates an imbalance in the force applied by the left and right brake calipers to the brake pads. However, this issue is not easy to detect because the brake disc rotates at a relatively high speed. 2. Weak braking: The cause of weak braking is likely due to a loss of pressure in the transmission pipeline that provides pressure. This situation is difficult to resolve on your own. It is best to drive the car to a repair shop for inspection. Otherwise, the consequences could be severe. 3. Brakes becoming too hard or too soft: If the brakes become too hard, it is likely due to a vacuum booster failure. This occurs with prolonged brake usage. Many components need timely inspection and replacement. Brakes becoming too soft is a more significant issue. It reflects insufficient hydraulic pressure in the calipers or master cylinder, possibly due to oil leakage, or a fault in the brake disc or brake pads. There may also be air leakage into the brake lines.
I've been repairing cars for twenty years and have seen many cases of the brake pedal slowly sinking at idle. The most common issue is internal leakage in the brake master cylinder, where the hydraulic fluid flows back into the reservoir due to worn seals. Another possibility is leakage from the wheel cylinders or aging brake lines, both of which cause insufficient hydraulic pressure. I recommend car owners first open the engine hood to check the brake fluid level in the reservoir. If it's noticeably low, it's likely a leak. If the level hasn't dropped, the master cylinder is more likely the problem. Whatever the case, don't ignore it—I've seen owners who delayed repairs end up with sudden brake failure while driving, which is no joke. For safety's sake, take your car to a professional repair shop for a thorough inspection as soon as possible. If the master cylinder needs replacing, just do it. The cost isn't high, but your life is priceless.
Last month, my old car had this exact issue—the brake pedal would slowly sink when idling. At first, I thought it was just weak foot pressure, but then the braking felt consistently mushy. I took it to my trusted repair shop, where the mechanic used a lift and found the left rear wheel cylinder was leaking. He explained this caused an imbalance in the entire brake system's pressure, leading to the pedal sinking. Spending just over 300 yuan to replace the cylinder fixed it completely. A word of advice: if you notice the brake pedal travel getting longer or sinking, don’t brush it off like I did initially. Brake problems only get riskier the longer you delay—address them early for peace of mind.
Studying automotive brake systems is quite fascinating. This is actually a manifestation of hydraulic transmission failure. When you press the brake pedal, the pressure generated by the master cylinder should be maintained. However, if the internal piston seals are worn, the brake fluid will leak from the high-pressure zone back to the low-pressure zone. Similarly, leakage in the wheel cylinders can cause the pressure to slowly dissipate. It's also common for brake fluid to have excessive water content due to long-term neglect of replacement, or for brake lines to develop micro-cracks from aging. It's recommended to change the brake fluid every two years and pay attention to any changes in pedal feel during daily use. Never take chances if you encounter pedal sinking – any issue with the entire brake system is a serious matter.
