moisture in the brake line

Why does moisture in the brake lines reduce the effectiveness of brakes?

The answer that is given from a number of sources is that when pressing on the brake pedal, the pressure goes up in the brake line causing the moisture to boil.

But that really doesn't answer the question.

As one presses on the brakes, pressure goes up in the line, the water boils which if anything ought to further increase the pressure and so additional force would be applied to the brake pad.

Whats the physics that is going on here........can the moisture as a gas be compressed further than the moisture as a liquid?

Reply to
kiselink
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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.

Reply to
M. Hamill

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.

Reply to
qslim

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

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  - Philip
Reply to
Phi-l-ip

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!!!)

Reply to
HachiRoku

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