If this is a factory system, as I was assuming, the brake turns off as soon as you begin accelerating. Then when you begin to decel, it turns back on.

On the other hand, if he has an aftermarket brake with a physical valve in the exhaust, it might be possible to have the valve closed at all times. This would cause major performance problems, just like any exhaust restriction will. I would imagine that driving the truck that way for a while could damage something, but I don't think your friend would do that.

James Kuhns
Owner/Technician
Auto Tech Solutions
Chatham, New York, USA