To avoid reading this diatribe - the upshot is fail to change the brake fluid and the caliper seal grooves will corrode and eventually jam the pistons. Crud between the seals on a BMW/Brembo caliper will also jam the piston but corrosion seems to be less of an issue.
Ive just rebuilt the back calliper on my 2008 1200GSA. Before I bought it the bike had BMW extended warranty and all BMW servicing. However the back brake was beginning to stick. I had already cleaned the pads and sliders but the pedal was feeling solid and the disc binding. Having had Japanese bikes with badly corroded calliper seal grooves, I decided it was time for some new seals. The pistons came out easily with a bike tyre pump and air bed inflator attachment but one was more stiff than the other. Thankfully there was no corrosion but fluid in the lower cylinder was darker and that was the one sticking. New seals and its all back on the bike with brand new fluid. I did not use the GS-911 but then I wasn't playing with the front brakes.
Does old brake fluid deteriorate? Dead right it does. Should it be replaced every 2 years? Who am I to argue. The cost is minimal for a fluid change or rather flush-through its not really possible to thoroughly change the fluid. A full strip every few years make sense to be sure the pistons are moving and there is no corrosion. Don't scratch the anodising !!!
The Yamaha Diversion 600 I bought in 2006 was 8 years old with 813 miles on the clock and garaged all its life. I changed the oil and flushed the brake brake fluid through. But after a few months the back brake started to stick. I freed off the caliper but after 100 miles on the motorway the back brake had become so hot the black paint in the middle of the disc had burnt to light grey ash. All signs of black were gone. The brake never showed any signs of boiling its fluid - but then it was new fluid. Thankfully, the disc survived. I now assume a binding brake needs a full strip. Simply freeing off the pads and sliders is unlikely to solve the root problems.
The Diversion brake pistons were jammed solid even after all that heat. It was close to being a scrap caliper but I managed to get the old pistons out using hydraulic pressure. 100psi compressed air had no effect. Corrosion in the seal grooves had mangled the seals and jammed the pistons. It was all cleaned up with red grease in the seal grooves and packed under the dust seals. The brake itself had minimal corrosion but hidden away things were not so nice. Had it been a front brake I probably would have scrapped the caliper.
My next bike was 900 Diversion. Again, the back brake needed new seals and had to be stripped every 2 years to avoid any build up of corrosion in the seal grooves. Doing it regularly was a pain but saved me the cost of new seals. I had Blue spots on the front which suffered much less with seal corrosion.
A 2001 (bought in 2007) Fiat Punto HGT. Soon enough a front caliper seized up. The piston was solid and had to be sent away for rebuild. The brake specialist said it was corrosion in the seal grooves. I also changed the rear wheel cylinders - at £15 each it was daft not to. Front brake hoses got changed when the caliper was done. Bought cheap, run cheap and I got the bill.
The Mk2 Punto HGT has a 1746 engine - not slow & mine got "properly" used.
The brakes never showed any signs of fading but piston seal grooves had corroded due to lack of proper maintenance. Iron is less fussy than aluminium but its not stainless.
My 2004 Cannondale mountain bike has Hope M4 hydraulic brakes. They use Dot 5.1 fluid which does not absorb moisture but the caliper pistons were mostly seized and a nightmare to remove. I had to bleed each one and use hydraulic pressure to force out the pistons. The calipers were in good condition but the metal pistons were a mess. New phenolic pistons and its now working fine. Will I be changing the fluid every 2 years? Probably not - Dot 5.1 is silicone based, but they wont be left for another 10 years like the first set.