I have my transmission out and will be turning it in as a core on a reubuilt transmission. I want to make sure the lines are clean when the new transmission goes in. What is the best way to do this? AutoZone has this, Has anyone used it before? Before the transmission came out, I flushed more than 20 quarts of fluid through the transmission. What a fiasco that was.... Anyway, when I drained the fluid to remove the transmission, the fluid was a nice clean, high dollar red. I only managed to put 30-40 miles on the fluid. Do you think the lines are ok? Could I install the transmission, fill it with fluid and leave the return line off and use the transmission to pump the lines clean? You guys have been helpful so far, and I want to thank you!

I would think that, that is more money down the drain, but maybe it's easier to do that what I'm about to suggest.Since the biggest area of possible contamination from residue build up, is the transmission cooler. I would disconnect the line at the top of the cooler, add a seperate line with a funnel attached and pour some diesel into the line until it runs out the line at the bottom of the cooler. I would then stop up the line so that nothing runs out and fill it back up with fuel. Let it sit for some period of time. Couple of hours overnight? Then let the fuel drain out using a cheap see thru filter to catch any residue, just for piece of minds sake. I would then take an air hose with a regulater on it set to maybe 10-15 psi and blow air thru the whole thing, connecting where you poured the fuel in. Until nothing is coming out of the lower line. I would then pour the recommended ATF fluid in that the rebuild people recommend, and use air the blow it thru cooler and out the bottom. I would only do this if I felt I absolutely should flush it. Somebody else will be along shortly, to offer another opinion I'm sure. It is also possible what I described is not recommended by the other mechanics on here for valid reasons. I'm just a shade tree kind of guy, the guys that do this everyday are the better qualified.