Ive been wanting an exhaust brake for towing for a while now but i dont realy have the money to be spending on a pac brake. So i built one! I have an air cylinder on it and everything is ready to go except for one thing, i need to know what size the relief hole is exactily in the brakes like the bd and the jake brake so i dont hold to much back pressure and blow my engine up. Thank you for your help, John

 

My 4" jac brand has a .400" hole. This brake is for a 8.3 cummins. I would guess one for a 5.9 would be around a .250" to .300" hole.