I'll take a quick stab at this. If water mixed with braking fluid did boil,
it would probably occur as a result of heat generated through braking. The
presence of water vapor would likely reduce the effectiveness of the braking
system, because some of the compressive force of braking would squeeze vapor
bubbles instead of move the brake cylinders and caliper pistons as desired.

When mechanics work on brakes, they bleed (remove) air from the brake lines
before finishing the job to make sure drivers are activating moving brake
parts during braking, rather than squeezing air bubbles.

Moisture in the braking system could also cause corrosion.

MHamill is exactly right. When water boils in the brake line, it turns into
gas. Gas is compressable. Liquid is not. Thats why brake fluid has such a
high boiling point.

Did you fail junior high school science class?  MUST have.  Increasing
pressure raises the boiling temperature of water ... not lower it.

On Sat, 25 Dec 2004 13:21:41 -0500, kiselink wrote:

Just the opposite. Liquid *cannot* be compressed! So, when you press on
the brake, with the help of the master cylinder, your are pushing directly
on the calipers and the pads.

If there is a 'gas' in the line (like steam) it *can* be compressed, so
you lose braking efficiency, the pedal travels further, and take more
pressure to stop.

Bleeding the brakes is a good idea, anyway (actually I meant to say
flushing). It gets rid of too much firmness as the fluid gets
contaminated, or too much softeness if there is water.

I flushed the brakes for the first time in 16 years on an '85 Corolla GTS
a few years ago! I could not believe the difference. It was like the whole
braking system was brand-new again.

Easy way: they make a bleeding kit you can get at any parts store, or do
this:

Get a plastic jar with a good screw on lid. A quart is good.
Get some tubing at a parts store that is the same inside diameter as your
bleeders. Buy about 2 feet.
Get some RTV.

cut the tubing into two pieces, one about 16-18" long, and the remainder.
Punch holes in the lid of the jar, or drill them, *just* smaller than the
tubing you have.

Push the longer piece of tubing into one of the holes so it is about 1/4"
or less from the bottom of the jar. Push the other piece into the other
hole. RTV if the tubing seems like it will come out.

Now, you can flush the brakes. Remove the wheel if needed, put the longer
tube on the bleeder and open. Get in the car and pump the brake a few
times. Add fluid as required (it will take at least a quart for this job!)
Pump some more and add until the fluid looks new. Close the bleeder.
Brake fluid is burnable, so if you know someone who has a waste oil
burner, save it for them.

Repeat on all four calipers/bleeders.

using this method, not only are you flushing, but you're bleeding as well.
The set up can also be used as a one-person bleeder after doing brake work.
Take it for a spin and enjoy the great pedal feel! (Um, start out SLOW
first, just in case you missed a bleeder!!!)