84 scottsdale vacuum switch on brake pedal?

[dbmx66]

I have a 84 scottsdale, ive noticed when my engine is at temp and i get on the brake it revs up 200 or so rpms, so i looked up underneath the dash and noticed theres a plunger type switch above the brake light switch. It has a 3/8 rubber line hooked up to one side of it. I cant follow where the line goes, without taking dash apart. Any idea what this is? Ive never seen one before. There wasnt one on my 78 big ten, my brothers 87 custom deluxe or a buddys 85 k5. Any ideas? Thanks