All reservoirs must have an air vent that allows atmospheric pressure to force the fluid into the pump
when the pump creates a low pressure at its inlet port.
The pans of many automatic transmissions vent
through the handle of the dipstick; others rely on a
vent in the transmission case. Transmissions must
also be vented to allow for the exhaust of built-up air
pressure that results from heat and the moving components inside the transmission. The movement of these parts can force air into the ATF, which would
not allow it to increase in pressure, cool, or lubricate
the transmission properly.