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In the left side (left is towards the rear of the bike) of the master cylinder reservoir, notice the crystals that have built up. These have completely clogged the fluid return port.

The fluid return port is a tiny hole in the middle of the larger, left (rear) depression. I cleaned mine out with a single strand guitar string. You can see the depression, but the return port is not visible. You can see it in the next photo.

Brake fluid not returning when the brake lever is released has been known to expand due to heat during driving, and lock the front brake. Anton Largiader (our excellent local BMW motorcycle mechanic) noticed that my brake lever had far too little travel. He showed me a wrecked GS that this happened to - clogged return port, riding built up heat and pressure, the and pressure eventually locked up the front brake. Luckily, I don't think the driver of the GS was badly injured, but the bike was pretty badly thrashed.

I should have discovered the problem earlier. When I replaced my front brake pads, fluid would not return to the master cylinder. The pistons moved fine, but pushing one in forced the other out. If I pushed both, they wouldn't retract. I thought that was odd, but chaulked it up to some odd German engineering. Wrong. As for the lever travel, I also figured that was just the nature of the bike.