I changed the rear brake pads on my BMW last week, following the steps outlined in this video.
Unfortunately the brake pedal now feels spongy. The pedal feel was firm prior to the brake swap, so it is definitely a sudden change and not a gradual degradation of brake fluid.
One thing I forgot to do was undo the brake fluid reservoir cap before pushing the piston calipers back with a C-clamp, so I suspect that the the sponginess is due to trapped air in the system since the reservoir was never exposed to outside air.
Or is it possible that the extra pressurization of the system via the C-clamp resulted in moisture absorption? The only issue I see with this reasoning is that the system was never exposed to outside air.