Quote:
Originally Posted by sugarwood
The bore itself is never part of the contact area. If there is a pitting in the piston, and fluid will bypass the seal there, and you lose pressure. Right?
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I would amplify/clarify the last comment regarding pitted pistons with the following points of consideration:
Pitting is from corrosion. Corrosion tends to expand beyond the normal diameter of the piston, which can cause dragging/seizing. A piston frozen in the bore is either not doing any braking at all, or is dragging all the time on the rotor. Either way, no es bueno. Both scenarios result in under-performing brakes. The latter scenario can result in exciting fun things ranging from cooked/overheated rotors, glazed pads, boiled fluid, total brake failure, fire, etc.
Usually either condition can be readily observed by using one's senses. Frozen pistons stuck at the bottom of the bore will result in reduced braking capacity and sometimes 'off' or changed pedal feel. Dragging pads can result in stinky brakes, smoke, reduced acceleration capacity, blue colors on the rotor, etc. There is also going to be a LOT more heat on that corner than there is on the others. The constant heating is what glazes the pad, ruins the rotor, and worst - can boil the fluid.
Pitting results in an irregular surface (obv) and can abrade/tear the fluid seal. This can lead to low fluid levels and eventually brake failure. I have never seen a situation where all the brake fluid got squirted out catastrophically at the bore seal, but it isn't unheard of - especially on older cars that are not equipped with a low fluid sensor - to run dangerously low on fluid. Study how the braking system works - the fluid reservoir feeds the master cylinder via gravity. We keep a reservoir so that the MC plunger(s) always have fluid available on the downstroke. If there's no fluid supply, the MC will instead pull in air. The air will compress, resulting in a squishly pedal and little/no braking force.
The dust boot is important, as it keeps brake dust and other crap from compromising the fluid seal. Think of the dust boot as the first line of defense. Also, track days - while they are generally harder on the overall braking system than commuting - can do a great job of finishing off dust seals that were already on their last legs.
Changing fluid is important, because conventional brake fluid is hygroscopic (it absorbs ambient moisture in the air). Since the brake system is not 100% sealed from the environment, it will, over time, take on water. That water in the braking system will begin to corrode the metals in the braking system.