The restricted leg of the "Y" connector goes to the vacuum supply. The other 2 legs go to the vacuum valve and the transmission. There must be a restriction in one leg otherwise the vacuum valve would not regulate the vacuum to the transmission. I learned about this from the rebuilder that sold me the rebuilt transmission.

The rebuilt transmission I put in works so much better than the original, it was almost worth the cost, ~$1200, to get the updated transmission.
~~~~~~~~~ P E H