I have Avid Elixir brakes on my mountain bike. They are the only part of the bike that's a total black box to me. I understand the principle behind disc brakes, and I understand the principle behind hydraulics. I could draw a diagram of how this all goes together. That said, I've been told that they automatically adjust for wear on the pads, so as the pads wear down, the braking action is the same. Part 1 of the question: Is this true? Part 2: How the heck does it work?
I think the answer he was looking for was how they adjust as the pads get worn. There is a check valve in the master cylinder, that will allow enough fluid from the reservoir when the lever is pulled. If more fluid is needed because of pad degradation, it passes it into the active system. Therefore with more fluid in the system, the piston is pushed out slightly more and the pad is the same distance from the rotor.
That is why your res. needs to be topped off occasionally.
Many high end hydraulic systems on bikes are very much the same design as the car & motorcycle variety. However there are cheaper cable/hydraulic type brakes for cycles avalable.
What happens in the fully hydraulic system is that the fluid within the system is put under pressure by you pulling on the lever, but the fluid and the system itself will not 'give' under this pressure as it is a pressure resistant system, apart from one part, the caliper.
Ok so.. Attached to this lever your pulling is what's called the master cylinder, this compresses the brake fluid in the system at the lever end, and forces it (in a small quantities) down the brake hose to the caliper.
As the master cylinder, brake hose and caliper are already full of fluid the fluid only has a couple of options.. It can try to expand into any gaps, or it can escape through any holes. However this is a safe braking system so we have no leaks and our fluid is trapped in our sealed environment :)
Ok.. The only place the fluid can actually move to, then expand in, is the calipers small chambers, these are located behind the caliper pistons and if the system has been bled properly should already be filled with fluid.
Each piston is fitted with fluid resistant rubber seals, each piston also floats inside the caliper, on these seals. The seals are very important as they not only help to stop dust/dirt entering the caliper and allow the caliper to float freely, but they also seal the fluid inside the caliper whilst withstanding the high braking pressures within the brake system.
In front of each piston is what you can see when you look down at the disk. You'll see the disk itself, the pads, and if you remove the pads you'll see the front end of the pistons. Now as you know behind each piston is the brake fluid, this is the brake fluid your are trying to make expand by pulling on the lever. Still with me? :)
The only thing this fluid that your pressurising (by pulling the lever) can do, is try and expand into the small chambers behind the caliper pistons which in turn pushes on the back of the pistons forcing them outwards, and against the rear of the pads. The pads then react in the same way and are forced outwards trying to squash the disc!
There is usualy no need for adjustment because as the pads wear down, the fluid just fills the remaining gap in the chambers behind the pistons. Although some brake systems do have an adjustment knob/screw on the master cylinder for adjustment purposes
Hope that helps.
Answer to part one:
Yes, within reason, it's true.
Answer to part two:
It works in a very simple fashion. Each time the lever is pulled, enough fluid to move the pistons, and therefore the brake pads, is pushed out of the caliper far enough to contact the brake rotor.
As they move out of the piston, they push past the caliper seal. The flex in the caliper seal acts as a retraction spring, to pull the piston back into the caliper by just enough to clear the rotor.
Since the seal that acts as a retraction spring is always in the same place relative to the piston, the pad wear is automatically compensated for because the pad is always pressed out of the caliper until it contacts the rotor.
It is a bit confusing to describe, but it is how it works.
Epic Bleed Solution did an amazing post explaining the details of hydraulic disc brakes, covering concepts and mechanic principles, different fluids, pads, rotors, etc. It's a very good introduction to hydraulic disc brakes that you can find here: http://www.epicbleedsolutions.com/blog/how-hydraulic-brakes-work/