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I don't think my question has any connection to vehicle maintenance or repair but I think this is the closest site to my question topic.

My question may be arising from misunderstanding. For that I'm going to assume a few hypothesis.

  1. The brake fluid is incompressible(which is correct) and there is no air in the braking system so it doesn't affect its performance.

  2. We're assuming there is no brake force boosting effects in the system for the sake of simplicity.

  3. If I press the brake pedal and it moves that means that the slave cylinders move.(correct me if I'm wrong)

Now my question is if the fluid moves when the slave cylinders move then

  1. Shouldn't the fluid be confined for Pascal's to apply?

  2. The brake pedal should stop getting further depressed after the slave cylinders press the brake pads and brake disk , is that correct?

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    All fluids are compressible, it is just not much and you need very high pressures. While in most applications it is negligable, in hydraulics it often is not, with pressures exceeding 100bar the compressibility can reach 1% which may or may not be a factor in a hydraulic application. Also we need to consider expansion of the confining containers sometimes
    – PlasmaHH
    Commented Dec 7, 2022 at 18:48

2 Answers 2

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You are correct, the brake pedal should only move when any of the 4 or more slave cylinder pistons move (some vehicles may have multiple cylinders in a caliper).

Once the pistons of the slave cylinders are pressing firmly against the brake pads, the brake pedal should in theory become very solid and not possible to press further. There will be some very small amounts of flex in some components, like the flexible brake hoses and even some flexing of the metal parts.

When the brake pedal is released, the fluid pressure is released and hence the slave cylinders are able to release pressure on the brake pads.

The piston in the master cylinder has a fairly small diameter compared to the diameter of the pistons in the multiple slave cylinders, so the amount of movement of the brake pedal is greatly amplified compared to the brake pad movement. This allows a small amount of pressure (but large movement) on the brake pedal to create a very large pressure (but small movement) on the brake pads. Without this you would find it very difficult to press the brakes hard enough to stop the vehicle. I presume this is the point you are questioning.

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  • So there is some brake pedal displacement that won't cause any braking force
    – John greg
    Commented Dec 6, 2022 at 20:36
  • Yes, there will be an extremely small amount of flex in components that could be noticeable as brake pedal movement under heavy braking.
    – HandyHowie
    Commented Dec 6, 2022 at 20:45
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    With now-obsolete drum brakes, the amount of movement at the wheels, to go from "not scraping the drum" position to "working well enough to stop the vehicle" is substantially greater than that of disk brakes, resulting in a corresponding increase in brake pedal travel.
    – fred_dot_u
    Commented Dec 6, 2022 at 22:26
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    @fred_dot_u There are plenty of drum brakes still in service, at least on rear wheels
    – Chris H
    Commented Dec 7, 2022 at 11:40
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    Same pressure in both cylinders - the difference in cross-section gives amplification of force.
    – Toby Speight
    Commented Dec 7, 2022 at 19:36
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  1. Put your foot just on the brake pedal (the brake pedal has not moved)

  2. Start moving the pedal in.

  3. the pedal moves - say - two inches before there is any braking.

If this two inches is what you are asking about ...

The answer is simple, that's the brake pads moving from not-touching in to touching the disk.

(Of course that distance is smaller, but it's leveraged on the pedal. Two inches of pedal travel equals the few mm gap between pads and disk.)

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    Just as an addition: Hydraulics can work like a set of gears. They can convert much travel with little force into little travel with a lot of force. If you had to push the break pad without that mechanical advantage, you wouldn't be able to stop the car. Your foot needs to press strong enough to produce friction strong enough to stop one or more tons of vehicle, moving at really high speeds (100kph+ times 1-5 tons is a lot of force). That's why the force needs to be converted, your foot moves a lot and the break pad moves only tiny distances, but very forcefully.
    – Dakkaron
    Commented Dec 7, 2022 at 15:37
  • @Dakkaron - for sure. I wasn't clear if the OP was asking about "why does the pedal move 'until the brakes hit'", so I tried to explain that! You rock
    – Fattie
    Commented Dec 7, 2022 at 18:21
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    Your answer was really good as well! My addition wasn't enough to warrant it's own answer, so I thought, I'd just attach it to yours ;)
    – Dakkaron
    Commented Dec 8, 2022 at 19:24

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