I would like to flush my auto by breaking into the line at my HD cooler which has screw fittings, I will use the 2litres out 2litres in method at the the cooler outlet.
I have been reluctant the do it this way because of posts on here as some say there is big pressures in the ZF's and some say they a not to sure about the pressure.
After a bit of googling i found this "field testing of an automatic [at] on an off-road vehicle" [sorry don't know how to add a link] which shows pressures from 300kpa/43.5psi to nearly 800kpa116psi thru different gears and revs on a 4hp24 in a P38.
I am no longer worried to change the oil this way, at those figures it wont be a problem with a few extra fittings and barbs as the motor only idles.
I hope you find it interesting.