[petrel]

When departing, I did not activate the engine brake until the truck was warmed up and we were idling down the street. At the first light (and all subsequent stops), the brake worked as it should.

My truck is equipped with a generic electronically actuated Hi-Low engine brake. Apparently it calibrates itself when it is first turned on. I guess that is why you want the engine warm and in gear for the first activation. After that, it doesn't seem to matter when you switch it on and off.