Best way to flush an automatic transmission?

[a6m5zerosen]

I have three cars that I need to flush the automatic transmissions and change their filters. The filter change is easy enough, but what's the best way to fully flush the transmision, pump, cooler lines, and cooler? Shops around here want between 80 and 100 bucks to do it, and that's just the flush-no filter change. It can't be that tough, but I wouldn't want to damage anything. The transmissions are a '71 turbo 400, a '77 turbo 350, and an '85 200-4R. Thanks for any input.

[Olds64]

Disconnect the return line from the transmission cooler. Then fit a piece of 3/8 inch rubber hose onto the fitting of the transmission cooler. While watching the end of the rubber hose have a buddy start the car and allow the transmission fluid to gush into a bucket. Run the engine for about 30 or 40 seconds until you see the fluid change from dirty brown to bright red (this assumes that you just changed the filter). Once the fluid is running clean stop the engine, tighten everything back up and check your fluid level. Don't let the engine run the transmission dry. It should only take about 30 seconds.

I got this cool idea from Ron Sessions' book "How to Work with and Modify the Turbo Hydra-matic 400 Transmission."