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It came off the master cylinder (the one on the bottom). I imagine it's either a proportioning valve of some sort or a pressure gauge. And why would there be a pressure gauge there?
http://i32.photobucket.com/albums/d3...psdmyyg95m.jpg
Here is the other side. It's mostly illegible, but it looks like there are numbers. Looks like a pressure gauge.
http://i32.photobucket.com/albums/d3...pse4ctxml1.jpg
The top one's a brake light switch, which is just a pressure switch to actuate the lights. The bottom indeed looks like a bourdon tube for a pressure indicator. Not sure why....
The bottom one is a beat-up, almost demolished bourdon-tube pressure gauge with case broken off.
FYI, if you mess with the bourdon tube it can break easily and you lose your brake fluid.
Seems to me it would be worthwhile to take that old gauge out and insert a pipe plug to avoid what Roger said.
That is one hella cool pressure guage, I bet it helped a lot of mechanics diagnose brake issues. It's kind of self evident why it didn't catch on. Dangerous if it fails.
So ugly it's kinda cute, huh? I had one under the driver side frame rail in my '56 GMC and since the brakes worked fine I never touched it.