Excessive amounts of ATF are needed for a thorough flush.
To understand how the ATF flush works, fill a labelled cup(glass #2 the ATF sump) to the brim with koolaid(dirty ATF).
Now, add a cup of water(glass #1 flush machine clean ATF) slowly overflowing the Koolaid.
Still have a pretty colorful cup #2.
How many cups of #1 do you need to overflow #2 with, before there is no koolaid???
And, that #2 is a single sump. Your tranny has 2 or more(TC, ATF cooler, screw on filter, hoses... are sumps also).
So, add another cup in series (glass #3 torque converter) of koolaid to the above test. Anything that spills out of glass #2(sump) is what feeds/flushes glass#3(TC). Notice how the effluent from glass #2(water koolaid mix) does a lame job of flushing glass #3.
You get what you pay for. Discount flushes/shops will cut as many corners as possible. Not changing the filter, not cleaning the magnets, not starting with a pan removal/cleaning or sump draining, not rinsing the ATF cooler, and my favorite, not filling the flush machine fully, will all add to a incomplete low quality flush.
A typical ~15 quart flush is acceptable in small transmissions. And, its even more successful if you can drain/refill the pan/sump prior to starting the flush.
Too many rumours start with "someone told me".
