In a completely normal working transmission the fluid level will rise as soon as you shut off the engine - this is due to the top half the torque converter draining out in the transmission.

How would this valve prevent that?

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)