Ok a few months back I drained and refilled my wife's transmission in her 04. I added the same amount that I took out and moved on to flush the fluid out of the cooler. I plugged the return hose and put a length of hose on the nipple at the bottom of the cooler. I started the car expecting fluid to come out into my drain-pan and nothing happened. Stopped and started several times still nothing. Is there a thermostat or something somewhere that would stop it from letting me flush the fluid out? I let it run for 5 minutes and still nothing.

Any ideas? does this procedure need to be done when the engine is hot?