The pressure activated brake switch on these old pieces is well known for both internal failure and external failure. Contaminants, corrosion to the outside terminals lead to the failure you initially experienced. Like ND said, internal crud leeds to failure of the switch. DOT three brake fluid being hygroscopic draws moisture which exacerbates the problem.
These switches should activate with very little pressure and turn off when the pressure is released. You want the switch to activate when you roll up to a stop with the brakes applied easily so as to notify following drivers of your intentions. It is also necessary that the switch turn off when no pressure is applied or you will have a dead battery. The switches are cheap.