I don't know about your IC Cummins but my C7 Cat has a Pac brake which is just a butterfly that shuts off the exhaust flow on the outlet side of the turbo. I bought my coach in 2008 which was a 2007 model with a 2006 pre-emission engine. I was always suspicious that the pac brake wasn't working. Last fall I removed it and it was set tighter than a drum. After a lot of heating, penetrating oil, and pounding I free it up. I lubricated it with the Pac Brake lubricant and watched it while someone drove the coach. It still didn't engage unless I tapped it with a hammer. On the activating rod there is turn buckle that is adjustable. I lengthen out out and It now works as when I brake my turbo pressure is 0 which it should be when it engages.If you have turbo pressure of more than 1 lb the brake isn't working. Now this only pertains to the pacbrake system. I don't think mine ever worked from day one as it was never adjusted right.
