We had a tranny flushing machine at school but it was more of a machine to pull all the fluid out so there wasn't any old fluid left. drop the pan change filter put 4 quarts in fill the machine and start the truck let it pump out and it automatically fills. That wouldn't be so bad unlike the ones that push the old fluid back through the transmission. Depends on the machine i'd ask and find out exactly how they "flush" the transmission. you could do the same by pulling the trans line and keep dumping more in as you run the old into a bucket till you get clean.