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.
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.
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.
I've been researching this question in order to be able to adjust my self-adjusting brakes ;) So I'll report my findings here. I can't vouch for the accuracy since I am no bike technician.
The pistons are floating freely and are surrounded by seals. If there was nothing holding a piston back it would be possible to pump the brakes until it popped out of the caliper.
Engaging the lever pushes the piston further out and releasing it again lets the seal retract, which will in turn retract the piston somewhat (enough to release the brake).
If you want to adjust this type of brake, all you have to do is to push the piston further out, i.e slide it out so that the seal sits further back on the piston body.
Under normal operation the piston only moves with the seal and does not move relative the seal. When the pads are worn enough, however, the pistons will slide futher out relative the seal, simply because they aren't held back.
As you already noticed, the system needs more fluid if the pistons slide out. This is solved by means of a reservoir. When the system is in rest and the main cylinder is retracted, the lines are in direct contact with the reservoir through holes called timing ports. This connection will neutralize any pressure differences and thereby fill the lines. When the lever is actuated, the main cylinder passes the timing ports. After this point the system acts like a closed one.
My problem was that the system didn't want to auto-adjust. The gap between the pad and the rotor on one side was a bit too large. What you can do in this situation is to over-actuate the pistons a bit (which will force them to slide relative the seals), and push them back to where you want them. This way they will sit better relative the seals, giving you the right range of motion.
This procedure is called "advancing the pad position" in old Avid instruction manuals. The new ones don't have it since they are supposed to auto-adjust (but don't always do). Google it and you will find detailed instructions. But basically what you do is remove the caliper from its mount, push the pads until they almost meet (leave about 1mm), push them back to fit the rotor, and re-mount (which will also re-center the caliper). Made a big difference on my brakes.
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/