So I want to flush my trans and the advice I got was to cut the top cooler line and connect a hose to a bucket and pump it out.

I just want to make sure that the upper line is the return so that I pump out the trans through the radiator.

 

Found this on a thread here, can anyone else verify it's correct? This looks like the upper is the feed so if I'm pumping out from there, I should push the old fluid down and out of the radiator into the trans pan (thinking I could use a little air pressure). Maybe I should cut the lower line and pump from there. Still contemplating.

As I understand it, these are 5/16" lines.

UPDATED. This diagram is WRONG. On my 89, the top line coming out of the radiator is the return.

 

why cut any line? just disconnect it, slide the fitting back, and put the hose over it.
you'll know which one is pumping as soon as you start the car.

 

Those connectors are a nightmare to tighten back up and not leak. At least for me they are.

 

Well that diagram is WRONG. For my car the feed (pump) was coming out of the radiator at the top. It blew my little cap off and pumped out about 2 quarts before I could stop it!! Once I got on right side, it worked great. Went through about 11 quarts of Mobil 1 synthetic ATF to flush out everything but now the trans has new blood.