When one of the brake circuits (front or back) fails, there is a piston in there that slides and illuminates the brake light in the speedometer

The fluid passes right through it.  Front and back are separated.  The piston in the middle moves if there's a pressure differential.

Wow, thanks Mike, Patcon, and Zed Head - super helpful posts. Now to find the wire that goes there.

Just as a follow up, new fluid and the heat shield did the trick. Brakes worked perfectly last track day.

When one of the brake circuits (front or back) fails, there is a piston in there that slides and illuminates the brake light in the speedometer

The brakes need to be “pumped” to actuate the piston in this device. The piston then blocks the defective line and shuttles all fluid to the working brakes. Quite ingenious really.
... as long as you remember to pump the brakes when they fail!