I haveswitches on the brake pedal andon top of the master cylinder. What's the one on top of the master cylinder for?

 

On top of the m/c? There shouldn't be anything on top of the m/c except the zinc-coated cap. There is, however, a switch on top of the distribution block. That's part of the pressure differential switch for the brakes; it lights up the dash brake light if one side of the brake system loses pressure.

 

64 1/2 mustangs have the brakelight switch on top of the master cylinder. maybe you got one of those?
disconnect the cable there and see if your brakelights still work/. Then you'll know which of the switches is used

 

looks like a 67/68 in the pic, though. Don't know why anyone would put a single-bowl m/c in one.

 

OK. My bad. It's a 68 Mustang, and it has a dual bowl master cylinder with two lines coming out of it. The switch is on one of those lines, not the top of the master cylinder. Sorry. So what have I got there?

 

make a picture sure.
They only ideas that come to my head are:
stoplightswitch, brake warning as Tad says and low brakefluid level warning.

Kalli

 

On one of the lines? Or on the brass block on the fender?

 

On the line that's closest to the firewall.

 

That switch is the for the Inst panel "Brake" warning ight that comes on when the shuttle in the distribution block senses there is unequalized brake pressure. But I don't see that circuit on a 68. It looks to start in the 69 model year. Is the switch wired?

 

Yes, it's wired. The switch is round, has two prongs on the top, and the wires go thru the firewall into the interrior.