I've been reading about transmission flushing machines and it looks like all they do is equal the pressure they inject new oil into the system to the pressure the transmission pump pumps the old oil out. I had an idea to build something similar more simply. It may not work, but I just exhibit here to know your opinion. Imagine a big syringe with the capacity of your transmission, that is the same in both sides, needle side. Then you don't have the common syringe piston, just the rubber part, but with a metalic core just like a Tea-Pot, running all along the syringe so the rubber piston doesn't twist. Then you move the rubber piston to one extreme and fill it with new oil. Connect the outlet side of the cooler line to the empty side and the inlet to the full side. Your start the engine and the oil starts to flow, pushing the piston and pressuring the new oil into the transmission with the same pressure as it leaves, so the oil level remains constant. You just have to be very careful to stop the engine before the piston comes to the end, so you don't put too much pressure into the lines. What do you think?, would it work?. Do you know if this already exists or if there is something similar that could be adapted to do the job? Another posibility would be to use a veeery long transparent hose that you would fill with new oil, then just connect both lines to it and wait till the new oil is into the transmission and the old is in the hose.