It is a very simple hole. No moving parts.
I agree it looks like a solid metal plug with a hole in the underside of the threaded end that continues out of the side of the plug,, the plug has a rubber ring washer and when I screw it in tight ,,,and then pump the brake pedal by hand ,, it squirts brake fluid out through the side of the plug where the hole is .. But after sticking a tooth pick in the side hole ,,it does not squirt fluid out when I use my hand and pump the brake pedal ,, I just don't get it ,, yet ..
