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I was driving home today when the brake pedal suddenly got stiffen, if i apply the brake suddenly the brake pedal is so hard that it wont go down enough to hold. I checked and find out my brake fluid was completely dried up.

I bought another brake fluid to fill it but, it doesn't change the hardness on the brake pedal.

I took it to a mechanic who decided to bleed the tires but, the brake pedal is still hard and could not hold when I applied the brakes suddenly to function test it.

(1) What is the wrong? (2) What should I do to resolve it?

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    Welcome to Motor Vehicle Maintenance & Repair! He told you to "bleed the tires" or "bleed the brakes"? Nov 2, 2020 at 22:04
  • I had a similar problem in an older Volvo. If I depressed the pedal slightly before I wanted to brake, then it would brake normally. But if I moved my foot from the gas pedal and braked quickly, the pedal would seize hard and not brake. I replaced the vacuum booster and the master cylinder.
    – John Canon
    Nov 6, 2020 at 2:43

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You didn't mention the car make and model, but I'm assuming your car uses a vacuum powered brake booster. (See below image) It uses vacuum power from the engine to multiply the force applied to the brake pedal. If the brake booster fails or if there is a vacuum leak, it will not work properly, resulting in a hard brake pedal.

enter image description here

The brake booster (black), the brake master cylinder (silver) with two brake lines and a vacuum hose from the engine providing vacuum pressure.

If a component in the power braking system fails, you will still be able to brake, but only with your own physical strength. This is a big safety issue, so you should never drive a car without a functioning power braking system.

(You can try this yourself: After driving a car, when you shut down the engine, you feel the brake pedal is still soft, but after you press the brake a few times without the engine running, the pedal gets harder, because all vacuum pressuse gets released when pressing the brake.)

I'm not quite sure what you mean by "the brake fluid was completely dried up"? If it means there is no brake fluid in the brake fluid resevoir (above the master cylinder, see above picture), you can have air in the system. This does not cause the brake pedal to feel hard, it causes the brake pedal to feel extremely soft and will result in a loss of braking power or cause the brakes to fail completely. If you have air in the system, you (or a mechanic) should bleed the brakes.

But again, I'm pretty sure the issue is caused by a vacuum leak. The brake booster gets vacuum pressure from the engine intake. Whichever part is leaking must be replaced.

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