Ok, most of the above is correct, but to clarify, regular brake fluid (not silicone) is hydroscopic, which does indeed mean it absorbs water.
How did water get in there?
It absorbs the moisture in the atmosphere, because the cap on the reservoir is vented - usually a little hole in the top - and that is all it takes.
Brake fluid in a well maintained and leak-free system will still get dirty over time, and if you have ever flushed a car out that has not been serviced regularly - I have seen brake fluid almost black - you will see a change in colour.
Again this dirt is absorbed with the moisture.
If there is 'crud' INSIDE the calipers, you have a major problem, and they will need a complete strip and rebuild with new seals.
I suspect you were refering to the stuff on the OUTSIDE of the caliper body, the dust and rubbish that collects behind the brake pads.
This is normal build up and can easily be washed down with brake cleaner.
It is worth checking there is no sign of fluid around the seals at this point, and certainly it is worth cleaning all the surfaces and the locating pins, as even a little build up can cause sticking pads or a spongy pedal as the pressure is having to compress other material before the brake pad moves, and this can cause longer travel.
Got those alloy wheel brake dust protection discs?
Your choice, and they certainly keep the wheels cleaner, but they also restrict airflow, and your brakes will run hotter, so, particularly if you are in hilly country, or tow a lot, get rid of them.
Another important tip is, despite what well meaning car maintainence books and NRMA advice guides may say, you should never 'top up' brake fluid!
The difference between the MIN and MAX marks on the reservoir are tha amount lost down the system due to pad wear - and hence the pistons are further along the caliper bores to compensate.
If fluid is low, it is for two reasons only;
1 - your pads are worn and need changing, and with new pads fitted the fluid would come back to the MAX mark again - except - you are changing the fluid at the same time anyway, aren't you?
2 - you have a leak - investigate urgently!
One other point, I have found a lot of garages and servicing outlets are now using the vacuum type bleeders, which suck fluid down from the calipers.
If it works for you, fine.
I have always used, and still prefer the pressure bleeder which attaches to, and pressurises the reservoir, for the simple reason that the vacuum type will only (with a good vacuum) one BAR, that is 14 PSI, which is often not enough to pull any trapped air bubbles, that will sit at the highest point in any bends in the hoses and pipes.
Apressure bleeder can often shift these at 25 or 30 PSI.
There are potential hazards with pressure bleeders though, so be careful;
reservoirs have been known to pop off master cylinders, due to weak locating grommets, and you have to be careful to disconnect the pressure supply BEFORE opening the reservoir cap, and then you have to syphon fluid out down to the MAX line.
The risk of spillage is I guess why the trade have gone over to the vacuum type.
And finally, I lost the pedal completely on my 280E coming down the MacQuarie Pass a few years back due to old (and subsequently boiling) brake fluid. I had only had the car a couple of weeks, but that is no excuse from somone who used to be employed by Midas (in th UK) twenty-five years ago specifically to train staff in brake servicing!
That will do for now,
Chris M.
