After replacing the brake fluid and applying the brakes, the pedal starts to sink automatically after the first foot is pressed on it, what is the reason for this? Is it the brake fluid?

Why does the pedal continue to sink after I release my foot? (Provided the engine is running)

After the first foot is pressed on the pedal, the outer air chamber valve of the booster opens, and the booster generates a pressure difference between the inside and outside of the booster, which generates a boost, at which time, even if you release your foot, this force is always present and is much greater than the force exerted by your foot.

The push rod in the center pushes the master cylinder forward under the action of the booster, and after the seal of the master cylinder spool exceeds the pressure compensation hole, the brake fluid is sealed off, the master cylinder spool can not continue to move forward, and the pedal no longer sinks.

       If the pedal can continue to sink, it means that the master cylinder spool to the pressure compensation hole position, failed to build pressure, resulting in the booster push rod to push the master cylinder spool to continue to move forward, the pedal continues to sink.

       So let's take a look, what would be the reason for the pressure is not established?

       1, first check the brake fluid is not leaking?

       2, check the brake fluid is not air intake?

       3, check the master cylinder seal is not aging wear?

       4, isolate each part in the system and check them one by one.

      In fact, in addition to these tips, we also need to know that a good brake fluid is needed to ensure that the normal flow of cold temperatures, so that we in order to be in the cold winter, so that the brake function is not affected in any way. And brake components, because of the presence of rubber and metal materials, brake fluid needs to be able to meet the function of the metal material rust prevention, but not corrode the rubber components. In addition, the brake fluid should also be resistant to high temperatures and evaporation, because the temperature when braking may reach two or three hundred degrees, so a good brake fluid is very important for the car.