Originally Posted by opensourceguy
All they will do is hook your car up to this machine, that will pump air through your tranny [i think] pushing the fluid out, while replacing it with new fluid. Or just drop the pan, drain refil [not a tranny flush].
Nope. The machine has a piston in it. The piston is all the way down when you start. Your old fluid is pushed by your own transmission pump into the lower chamber, driving the piston up, which has new fluid on the top side. As the piston rises, new fluid is forced into your trans. No air involved at all.
Air would be really bad news for this type of service.