How to judge brake system failure through foot sense?

First, we need to know in ideal conditions what the normal feeling is on the brake pedals. In short, it is linear, that is, when a driver steps gradually harder on the brake pedal, the braking force will gradually increase too, which is in line with each other. This alignment is kept consistent all the time, without changing along with the vehicle's speed, engine ed or climate, etc.

If it feels a little soft on the brake pedal, just like stepping on cotton without force feedback, then the most possible reason is too much water in the brake fluid, leading to gasification of brake fluid in temperature. As we all know, gases are liable to be compressed, leading to this forceless feeling on brake pedals. Beyond that, this foot feeling can also be caused by leakage of brake fluid. It is noticeable that the leaking out of brake fluid can be easily detected, but it will be difficult to detect"inner leakage", which is caused by the sealing problem from different working chambers of brake master cylinders, and brake fluid flows from the high-pressure chamber into the low-pressure chamber, making braking force fail to act on brake calipers.

If it feels stiff or even hard to press down when stepping on a brake pedal, then the problem is likely to be poor vacuum booster concerned. It is well known that most brake systems of passenger vehicles today are equipped with vacuum boosters, which magnify the braking force from drivers many times. Once vacuum boosters decline in boosting because of damage to vacuum pipes or diaphragm control valve failure etc., it will be hard to press down the brake pedals then. Moreover, it is notable that vacuum boosters rely on engines to provide vacuum, and it will be quite normal if a brake pedal is felt stiff when the engine is in a state of flameout.

Sometimes, it is likely to occur that brake pedals rebound at braking, also known as "knock foot", which is known by the drivers with many experiences as actually because of the involvement of ABS. This "knock foot" usually happens at emergency braking or on ice-snow pavement. If this problem takes place in regular brakings on normal pavement, it indicates there is some problem with ABS then. In most cases, it is the problem of some wheel speed sensor that mistakenly takes the wheels as being locked. In addition, it is also likely that something is wrong with the ABS valves or control system.

In normal cases, at the first section of stepping on a brake pedal, there is almost no braking force, which is also known as "free stroke". This is not a fault, but because of the structure of the brake master cylinder. On the contrary, it would be a problem if there was no free stroke for a brake pedal. The problem is normally caused by too many impurities in brake fd, sweet and deformation of sealing parts, or blocked compensating holes in the master pump valve. In addition, when a vacuum booster pump is not properly installed or the model does not match up, there is also this kind of problem.

Warm Reminders from SY-PARTS

It is suggested for both drivers and maintenance men to note the foot feeling on brake pedals. No matter whether it is soft or stiff on the brake pedals, are or anything else abnormal, always indicates potential safety risk, which may lead to a decline in braking performance, extended braking distance, etc., and then it is suggested to have the vehicle checked as soon as possible.

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