We have a Pak brake on our 2007, 5.9L, 3500 dually. It helps immensely but kicks out at 22 mph. I've been told that you can put a torque converter lock-up switch in and when in use, it will allow the brake to work down to one or two mph.

Has anyone done this?

 

I've got a lock up switch on my truck but did it so I wouldn't have to pay the $$ for an exhaust brake. If I leave mine engaged, it'll stay locked up to 0mph but it'll kill the motor and possibly tear something up. If I were you, I'd be content with the Pacbrake and forget about a lockup switch...you might regret it.