[Murco]

MASTER CYLINDER - The master cylinder is the most important part of your car!!! Think about it, it's true!
Look inside your master cylinder reservoir for a moment and look at the fluid. Does it look like apple cider, tea, or coffee? It should be a golden clear color. If it's slightly darker (like tea) it's time for a change. If it's really dark (like coffee) and there is sediment at the bottom (wet black film) you have extensive seal deterioration and need a new master cylinder, PERIOD!
How about green fluid? Never seen it? Ask a neighbor who owns a 3 year-old ford if you can see his master cylinder. If he's like 90% of people, it's the OEM fluid and I'm willing to bet it's green! Green fluid indicates the copper in the brake fluid has started turning acidic from excessive moisture and is eating the brake-line inner coating and corroding them from the inside out. Fords do it quickly but after a dozen years or so I'll bet some of our board member's cars have this too.
Getting back to the blackened fluid - That film is particulates from the seals coming apart. Our master cylinders are made up of;
2 pistons (primary & secondary)
2 chambers (primary & secondary)
2 primary seals (named for pressure sealing, not location)
2-3 secondary seals (used for separation and sealing)
2 springs (primary & secondary piston return)
and a housing and reservoir
If one of the secondary seals goes bad you will still have a decent pedal but your brakes will wear one or the other end much faster than the other. You can usually identify this problem when opening a master cylinder cap and finding one chamber lower than normal and the other near overflowing. Replace the master cylinder.
If one of the primary seals is dead you will still have brakes but with a low pedal and little stopping power. Replace the master cylinder.
If one or both of the springs are losing tension the pedal will return slowly and drag the brakes after you release pressure. Replace the master cylinder.
Also, consider replacement if you have headers and more than 50K miles on your master cylinder. The extra heat will damage the springs (heat-stress) and seals (hardening) inside the master. Our one advantage to cars is the relatively large expanse under our hoods so the exhaust heat isn't directly under the master cylinder.

ABS Pump/Module - There have been 2 different generations of the ABS unit in our vehicles over the years and fortunately they have been pretty good as far as reliability and maintenance. However, you don't want to have too replace one as the sticker on one of these bad-boys will make you cry! When you have your brakes bled have the tech use his little handy OBDII reader to cycle the ABS pump to get all of the fluid flushed through it. Other than that it's nothing you need to do much about for maintenance.
Got nasty fluid in the master or the ABS system? Guess what else needs replacement?!
Remember, gravity does it's thing with particulates too so when you find it in the master cylinder/prop valve/ABS unit you'll certainly find it in the calipers/wheel cylinders.

Their are 1000's of "professional" mechanics who don't know squat about proper brake repair, and I'll back that up. Think I'm pulling your chain?? The first post in this thread contains more info than is covered in an ASE MASTER CERTIFICATION exam for brakes, all 25 questions of it!!! Think about that!!!!