Unless you have silicone brake fluid, regular brake fluid is very hydroscopic which means it absorbs water vapor from the air. That is why you don't leave a bottle of brake fluid uncover once you open it.

If you can polish out the rust you can probably reuse it, but if you have pit marks on the piston you can't reuse it. For peace of mind, I normally don't reuse pistons that have rust on them. For my PORSCHE I installed stainless steel pistons.

When you pull the wheels you should find your calipers were made by ATI so buy all those brand parts. They will save you 50-60% over supplied dealer parts which is the same brand difference box.
Example is the caliper guide pins Dealer $23. ATI PARTS $10.50.

When doing brakes at 100 k replace all the wear parts you will sleep better.
Pins, guide pin bushings (use a little armor all and they snap in), retaining clips.
Pull out the bleeder screws and clean the cavity.
You can clean out the piston bore with a green Scott brite pad and some brake cleaner. Check the piston and bore for scoring.

What causes the caliper to go bad anyways? (Other than rust on the piston) Does the seal inside the bore swell up and cause the piston to contract and retract more difficult?

Bleeding process
 

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. :thumbup:

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.

PM sent

Update: Being a Southern California car has it perks lol pistons on both sides were rust free and had virtually no marks on the sides and the seal inside wasn't swelled up or anything so I don't know why the passenger side pads wears out a lot quicker. But my god the boot to protect the piston was a pain to tuck into the bore. Took forever but got it in nonetheless. Flushed the system and wished I had a power bleeder but it is what it is haha. Especially after having a little scare thinking I damaged the seals in the master cylinder. But I checked my brake pedal again and test drove it for about 3 miles of braking and hard braking and whew all is good! Thanks for your help guys!

Congrats on a job well done!:thumbup:

Did you clean and Lube the shuffle pins? These are the two stainless steel pins or screws that hold the caliper to the caliper carrier. 9 times Outta 10 I find people don't clean those and put a little high heat Lube on them and that in turn makes pads wear unevenly.

Check the calipers slide pins.

I believe BMW specified not to grease the slide pins.