Why can't I press the foot brake when the car starts? Why can't I press the foot brake when the car is turned off?

Brakes are very important parts of cars. There are hand brakes and foot brakes. The foot brake is available in every car and plays the role of emergency braking. Some cars need to step on the brakes to start, but they cannot be stepped on before starting. Why is this?

Why can't I step on the front brake when the car starts?

When you start the car for the first time, the foot brake is very hard and you cannot step on it. All automatic transmission cars have this problem. Manual transmission cars also have hard brakes before starting the car. These are all normal.

This is because when the car is first started, the vacuum booster pump of the brake system cannot assist the brake caliper because the engine is not started. As shown in the figure below, the brake caliper hydraulic cylinder is pushed by the force of the foot, which is very laborious because there is no force. Therefore, the brake pedal feels very "hard" at this time.

At the same time, since the brake fluid does not flow back, the more you press the brake, the harder it will be. This is also the reason why you only need to brake once when the engine is turned off.

When the engine is started, the vacuum booster pump starts to work and plays a role in boosting. At this time, stepping on the brake pedal will feel very easy and "soft".

Why can't I press the brake pedal when the engine is turned off?

The automobile braking system is mainly composed of the brake pedal, vacuum booster, brake master pump, brake auxiliary pump, brake disc, brake disc and brake oil pipe. When the brake pedal is pressed, the vacuum booster pump drives the piston of the brake master pump to move, generating hydraulic pressure, pushing the piston of each wheel brake cylinder to move, keeping the brake caliper on the brake disc and generating braking force to slow down or stop the car.

During the entire braking process, the vacuum booster pump plays a powerful auxiliary role. The power source of the vacuum booster pump comes from the engine. When the engine is turned off, the vacuum booster pump loses its boosting function. It is difficult to push the piston of the brake master pump by the force of stepping on the brake pedal alone, so we will feel that the brake is very hard, and we can't step on it to the bottom if we step on it a few more times. In other words, the foot brake is useless.

Similarly, the steering wheel cannot be turned when braking.

Can the brakes still be used if they are out of fluid?

It is impossible for a car to brake normally without brake fluid. In other words, it is just that the level of the brake fluid tank is low, which means there is brake fluid. If there is no brake fluid in the tank, there is definitely no brake.

The hydraulic brake system pressurizes the system through the movement of the master cylinder piston, and then causes the slave cylinder to open the brake. Under normal circumstances, the system pipeline is all hydraulic oil. No air is allowed in the middle. There must not be water. The main component of hydraulic oil is castor oil, which has good corrosion resistance, low temperature fluidity and high boiling point performance. Even if the slave cylinder temperature is high, it will not vaporize to produce air blockage and cause brake failure. If there is too much water, it will vaporize to produce air blockage.

If the brakes still work after they run out of fluid, there are three reasons to consider.

The brake system is leaking oil. This is the most dangerous situation and must be repaired immediately.

When the brake pads or discs are worn out, the brake oil in the oil tank will automatically be added to the slave cylinder. At this time, the liquid level in the oil tank drops a lot. We should pay attention to it at ordinary times. If the liquid level in the oil tank drops a lot and reaches the lower limit, it is likely that the brake pads need to be replaced.

The power pump is leaking oil inside and outside. It is possible that after the power pump cup leaks oil, the brake fluid enters the power pump. At the beginning, the brakes are normal. As the oil leaks more and more, the brakes will become heavier. When the oil tank leaks out, air enters the brake system, causing the brakes to fail.