It goes to the 3-1 vacuum connector, IIRC its for the HVAC system not the transmission.
Unplug and block the whole 3-1 and see it it makes a difference. then unplug the reservoir line by it self.
On mine I used a vacuum pump to check the lines connected to the 3in1, found one bad connector, only thing I was not sure about is the reservoir lines, as one holds pressure and the other doesn't, is it designed this way?