ATF Cooler - Direction of flow?

[Spetz]

Hi guys,

I installed an auxiliary cooler on my car with a thermostatic bypass valve.

This is the routing:
Transmission--> Bypass valve --> auxiliary cooler (if ATF is hot enough, otherwise straight to the next step) --> OEM radiator ATF cooler --> Transmission


My issue now is I don't know which is the ATF to/from lines on the transmission.

I disconnected both and started the car in P to confirm however ATF flowed out of both lines.
I assume one line must have more pressure than the other which dictates the direction of flow.

Is there a general rule for this?
The cooler etc have all been installed a long time ago by a transmission shop but I am curious to double check everything since I disconnected everything for different reasons

 

[Greasymechtech]

Was the transmission fully hot or cold?

 

[Spetz]

I believe hot, but I am not sure as I don't remember when exactly I did it.
Does that make a difference?

 

[Trav]

This is the return line.

 

[Spetz]

What is the technical reason as to why both sides pumped oil out if anyone knows?