If I am not mistaken there are two draindown valves in some of these vehicles. One near the heat exchanger by the engine and another in the cooler line. If you removed one valve - you can remove both lines from the trans and have no problem blowing either direction. If you can't you still have a valve in the system. As for where the fluid drains to - if it has a valve it is not supposed to drain appreciably. You will still get some drainage through the pump rotors, some through the lines at shut down. But I was told that the cooler as it cools actually causes a slight vacuum. With the valve you end up with an air lock. The valve is supposed to open at 1 psi - more than the air lock can overcome.