I had the original Bargman metal cylinder breakaway switch on the front of my '78. Looked just like dominicjohn's picture, here:
It developed a straight short to ground somehow (!), and I took it apart to see if I could just fix it.
I didn't keep track of it well enough to take a picture of the inside, but it looks just like you'd think it looks. There's two metal contact leaves that are separated by a plastic rod - which is attached to the safety cable. When the safety cable is pulled, the plastic rod is pulled out and the leaves are supposed to touch, sending full battery voltage to the trailer brakes, and slamming them on full.
The key word there is, of course, "supposed." When I drilled out the two little rivets and took the cover and rod off, the two leaves never did touch
. Apparently, they had been separated for so long, and through so many hot and cold cycles, that the springiness had left the building.
The two leaves did not make contact when I removed the pin.
Just about the last thing you want if (God forbid) your trailer ever pops off the hitch is for it to pass you going downhill because you have a Broke Dingus Breakaway Switch.
I just ordered a new Bargman switch off some popular auction site; I'm sure our supporting vendors have them as well.