Brake fluids incompressible or low compression?

[J]

I’ve heard that brake fluids are not compressible (and liquids in general) or hydraulics wouldn’t work. But I’ve also heard that they do compress but have low compression unless we’re talking about massive pressure.

For a typical automotive brake system on an automatic transmission, you can apply brakes until you stop the car from moving. But that usually occurs about 1/2 or 3/4th of the travel. There’s still quite a bit of pedal travel that can occur. If the car has stopped at 1/2 or 3/4th travel that means the pads have grabbed on to the rotors right? So where does the rest of the travel come from? Is it the fluid compressing due to high pressure from power brakes? Is it some kind of spill over in the vacuum booster for power brakes? Is it some kind of spill over inside the master cylinder?

This is assuming that there is no air in the lines or worn seals in the master cylinder. On a brand new car for example I have never observed a brake system where when the pedal is applied and pads meet rotor the pedal stops abruptly. I’d imagine that would feel uncomfortable. Instead the pedal continues to go down a bit until it stops (almost like the pedal can be mapped to a gradient). This is the behavior I am trying to discover the cause of.

Here’s a service manual that describes this:
http://civic.hondafitjazz.com/A00/HTML/00/SNB6E00D14300000000FGAT00.html

It says that the pedal can be expected to sink up to 10 millimeters after the pedal has stopped the AT from creeping and that this is normal. If the fluid is incompressible then what does the 10 millimeters of pedal travel represent?

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[Ben]

If i understand your question properly, i will give a brief explanation on how i understand it.

The way hydraulics work is you exert a small force over a large area to cause a large force over a small area.
Pressing on the brake pedal 10mm may only make the brake calipers/wheel cylinders extend a fraction of a millimeter!

There will also be “flex” in your brake parts. The calipers will flex as you exert hundreds of PSI of pressure into the pistons.
Your brake drums(if equipped) will also slightly “stretch”.
There will also be a small amount of expansion in any rubber hose in your brake system.

[Nicholas Clark]

The only Brake Fluid that is compressible is DOT 5 brake fluid, which is silicone based.

[Author]

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