I've heard lots of people if they waited to long then flushed there tranny crapped out shortly after. I usually just dump what's in the pan every 6 months. I guess if you have that much junk floating around in there you don't really have any other options
there are all kinds of problems that can happen with a flush- and it is likely a 50/50 split between improper flushes, and what the flush actually does..
some folks flush by reversing the flow of the fluid.. that isn't a good thing.. it presses seals differently, and it dislodges material that has found a nice little hiding spot.. and, it pushes crap into the guts of a transmission from the filter by reversing it's course.. that is how lazy techs at oil change joints do it most often.. they don't even remove the dang filter first..
the way that flushes jack up already goofed transmissions is by removing the clutch material from the oil that was actually helping friction.. if a seal is about to give, a good way to push it on out is by using a high pressure flush..
OP- about your flush... here is how you -safely
- do it to prolong the use of your transmission...
- buy a case of fluid.. by a filter.. drop pan, swap filter.. put pan back on, fill it through the feed tube..
- disconnect the low pressure line from the radiator.. point it towards a bucket..
- open every bottle of fluid and have them handy, staged within easy grasp..
- stick your funnel in the fill tube..
- chock the wheels, set the emergency brake.
- have a friend behind the wheel, with their foot staying on the brake, start the engine..
- you start pouring fluid in while the engine idles and your friend slowly goes from gear to gear..
- when you see nice and clear fluid coming out of the low pressure line, go another quart or so longer.
- tell your friend to kill the engine..
- button up the low pressure line..
- drop pan, install another new filter...
- have your friend start it up, and you look for leaks, and check fluid level.. top it off ..
there you have it, a poor mans flush, that is about 99% as effective as a hot flush, and 5000+% better than a reverse flush by some jerk off tech at a oil change joint..