A couple of things are happening, One when you apply the brake the lights will come on, when you let off the brakes go out, until the rear shoes get pulled back in by the springs, and the brakeline gets pressured up enough to set off the brake switch again. If you have a inline residual valve, and the brake switch is in the line between it and the rear brakes, that will do it. If so, move the brake switch to the masterbrake side of the residual valve and you should be ok. Key word "Should". "Chrystal ball is a little foggy at this kind of a distance." Rob