"The transmission is another story however. In order to do it completely -- including the torque converter -- you need to attach the tranny to a special machine that forces all the old fluid out of the entire tranny. It then replaces it with fresh fluid. This is usually not something you can (properly) do yourself."
ALLISON SPECIFICALLY CAUTIONS *NOT* TO DO THIS. Apparently a pressure flush can damage internal O-ring seals. The proper way to "flush" the entire transmission and cooler, by disconnecting a cooler line, has been stated many times here. A search will find it for you.
2001 2500HD D/A CC/SB 4x4<br />A.R.E. Z-series topper, CA 75-series boards, Bedrug, Fumoto valve, Amsoil air filter, Jan \'02 PCM updates,Transfer Flow 50 gal. crossbed aux. fuel tank, Bilstein shocks, 25 ft. Tahoe Transport toybox (9.6K loaded), Hensley hitch, Tekonsha Prodigy brake controller.