Usually it is just trash inside the seal area (rust, dirt, brake dust) and age of the rubber parts as they fail. A good rule of thumb is to replace them every time you replace the pads. Most people do not do that and based on my own experience you can get away with it as long as the brake fluid is changed frequently as the moisture gets pushed to the caliper and boiled every time the brakes get hot from hi speed stops thereby rusting up the caliper bores. Heat extracts the water from the brake fluid as its boiling point is 212F and most brake fluid is a min 2x of that. The front are much worse than the rear as 80% of hi speed braking is handled by the fronts.

Tips: DRAIN THE MASTER CYL FIRST (suck it out) Flush the lines and system fully with new fluid (buy a pressure bleeder it will make your life easier). The process is RR, LR, RF, RF. The cheating method is to flush out the master cyl and ABS pump by bleeding out the LF first then go to the normal sequence. This step saves you the effort of pushing all that old stuff all the way to the right rear as that position will take a while to run clear. On the final drain, Tap every caliper with a rubber mallet to get the last bits of water/air to release and flow out. I personally, flush with a power bleeder all four corners than come back and do it second time, then triple check with a foot pressure bleed to ensure everything is clean and operational before a road test.

After bedding in the brakes, I re-bleed once again to ensure that nothing is left to chance. (One too many anal steps I am sure for any shop situation). Having seen way too many shop mistakes in my life, I leverage Best Practices only.

Brake fluid boiling points

Boiling point ranges
Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)

PS. If you were on the East Coast I would say come by and we can complete this in an afternoon wrenching session as the weather today is great and dry.

__________________

2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green