Gary,
The Jake or retarder will engage anytime you lift from the throttle. And the brake lights will then come on. That is fine if everytime you are lifting from the throttle you are braking, but if you are on the interstate and you or your cruise control lifts from the throttle only to maintain a given speed because you are going down a barely perceptible grade you will drive those behind you nuts because they will think you are riding the brake.
If you want to know how truly disconcerting this can be, follow another coach that has not pulled that relay, and that has the Jake or retarder engaged. After a while it will either annoy you or piss you off.