<<A 'Flush' is a waste of money! That's just a new term they use that makes a fluid change sound more expensive.>>
Maybe in some cases, but not most.
A regular, old style trans fluid change involves (on most cars) dropping the pan and changing the screen or filter, and refilling. On a Taurus, for example (since it's a popular car and I know about it) this will replace about 4-5 quarts of fluid. The trans holds approximately 13 quarts. So you have 8-9 quarts of used fluid left in there, and changing the filter really didn't help much.
A trans flush machine flushes ALL the old fluid out while the car is running, in addition to running cleaner through it. This is MUCH more beneficial than changing the filter using the old method.
That's why you can change fluid the old way and restart the car, and the fluid will still look dirty, but when you use a machine the fluid looks just like it came out of the can even weeks later.
We were all skeptical when we were told we were getting machines, but after they were demonstrated, all the tranny men said the machines were going to cost them a lot of money(in lost overhauls), but would be great for the customers.
Another example: To demonstrate just how well they worked, the rep asked to try a flush on a known bad trans. We happened to have an Aerostar that the tranny was smoked in. It would barely even pull. It was going to be rebuilt anyway. So he hooked up the machine and flushed it. Before, the fluid was black as soot.
The tech removed and disassembled the trans......it was as clean as if he had washed every part.
Have your tranny flushed, it's well worth it.
<<Some fluid will remain regardless of the technique used...it's there to lubricate! You would never want it to be completely gone. >>
Tom, after the cleaner is run through, the old fluid is sucked out while new is put in. No old fluid remains, except maybe residue.