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.
None of the answers here was satisfactory to me, but one had a link having image that explained it all: https://web.archive.org/web/20210507014839/https://cdn.sram.com/sites/default/files/techdocs/gen.0000000004234_rev_a_hydraulic_disc_brakes_overview.pdf
The idea is that the brake levers have a master cylinder. The master cylinder has a reservoir of brake fluid and a piston going towards a brake line. When the piston is fully retracted, it is outside the brake line so the fluid in the reservoir is connected to the fluid in the brake line.
When you start to push the brake lever, the piston goes inside the brake line, disconnecting the reservoir from the brake line, and starts to push the brake fluid in the brake line.
At the other side of the brake line is the brake caliper. It has peculiar seals. The seals are made from rubber and are rectangular. However, the metal container of the seal has one slanted side. Even at the danger of doing a minor copyright infringement, I took a screenshot of that SRAM document:
When the fluid (yellow) is pushing the piston (intermediate grey), the piston seal (dark grey) bends until it is bent in such a manner that the piston seal right side is touching the slanted right side of the seal container.
This bending of the piston seal causes the piston to move the brake pad on that side towards the rotor. If the brake pad doesn't have any wear to adjust, now it is touching the rotor and all is good.
However, if the brake pad is not yet touching the rotor, then the massive pressure in the brake fluid causes the piston to slide on the seals. Friction at this interface is large, so this happens after (not before) the seal has flexed. The piston slides on the seals until the brake pad touches the rotor.
Then, let's assume the piston has slided on the seal (self-adjustment has happened). When you release the brake lever, it creates a vacuum pressure in the brake fluid. This vacuum pressure causes the piston seal to bend back to its original rectangular shape so that the piston moves back from the discs. But because self-adjustment has happened, this is not enough to remove vacuum pressure from the brake lines. The piston would need to slide back on the seals. However, that would require so much force that the vacuum pressure is incapable of doing that. So this creates a vacuum in the brake lines, there's lots of brake fluid and a small pocket of vacuum, nothing, not air, not anything.
When the brake lever is fully retracted, in the master cylinder the brake line is reconnected to the brake fluid reservoir. This causes new brake fluid to fill that vacuum that was created in the brake lines.
You are back to square one, except self-adjustment has moved the pistons and pads automatically towards the rotor, and new brake fluid has gone from the reservoir to the lines.
Because the brake pad retraction amount is determined by the slanted container of the brake piston seals, you can't adjust it. Not unless you disassemble the brake caliper, find a Dremel and start grinding the slant to a different angle.
The metal spring spreading the pads isn't the component that causes retraction. It's there only to maintain full contact between the pads and the pistons as the pistons and pads retract. It doesn't cause the retraction itself.
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.
Self-adjusting for pad wear happens only for open system hydraulic break type, not for closed. The self-adjustment is not an outcome of any hydraulic system, it's an outcome of specifically the reservoir in a hydraulic system.
An open system contains a reservoir partially filled with air and partially with oil. This reservoir is disconnected (sealed off) from the rest of the system when brakes are applied, thus hydraulic force transfer can take place. The reservoir is re-connected to the system when the brake lever is released (near its idle/resting position).
Open systems are still sealed. The total amount of oil in the system remains constant, but the volume (and thus pressure) of that air varies as pads wear out.
When pads are new and the slave cylinder at the caliper are in their most retracted position, the volume of that air in the reservoir is smallest. That is, by manually pushing the cylinders inwards, you are compressing the air in the reservoir. When pads are worn and the slave cylinders are in their most extended position, the volume of that air in the reservoir is largest (thus lowest pressure).
The reason it is possible to push the cylinders inwards manually, without them popping back out, is the friction in the cylinder seal. To move the cylinder outwards/inwards that significant friction must be overcome. And, to compress the air by a tiny amount (displacement of the cylinders is small, a few millimeters) requires a force that is smaller than the friction; thus the friction holds the cylinder in its retracted or extended position, even though technically there is a small force from that reservoir air that is constantly acting on those cylinders.
This does mean that on that first application of breaks, after a new incremental pad thickness has worn out, when the breaks are engaged, the high force not only moves the slave pistons normally, but also pushes them further outwards (overcoming friction). After this happened, the master piston would not return all the way to its starting position -- it would stop short by exactly the distance that can be calculated from distance the slave cylinder pushed outwards and the ratio of the respective diameter of the pipes, keeping the volume constant.
To make the lever retract all the way to its starting position there are only two ways: either the slave cylinder is pulled back in (in principle, in a closed system, that would be possible by forcing the lever away from the handlebar -- this motion which would in principle attempt to create a vacuum, but of course the Earth's atmosphere will instead push the cylinder back in rather than allowing a vacuum) OR new liquid is added into the so-far closed system.
The latter is accomplished by ensuring that the reservoir gets re-connected to the system when the lever is in that not-all-the-way retracted position. The force from the backward motion of the lever sucks oil out of the reservoir instead of pulling the slave cylinder back in. Sucking oil out of the reservoir is only possible because there is air in that reservoir and the volume (i.e. pressure) of that air is allowed to decrease.
This is not at all simple. The only detailed (though not detailed enough) explanation I could find, tther than the brief answers here is in this document by SRAM:
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/