Can you post a picture? You want to make sure you get your feed from the correct line.

https://mail-attachment.googleuserco...8OExn48jtqD0xg

Here is a pic of the Y block, 2 lines and 1 line out. I have no idea why it's set up like this.

You need to use the attachment option for the pic to show.

disconnect one line...see which one pours fuel.

I'd get rid of that and go one line to the pump.

Air leak
 

It sounds like you have an air leak somewhere along your lines. try to see if you lines alone will hold a vacuum. Disconnect them at the rear sump (obviously, cap those), then see if you can pull a vacuum on your system. The interior of the lines/hoses behave differently under a vacuum than under pressure.

My customer's 66 'Stang had a short line (1yr old) that was collapsing inside & doing just about the same thing, but at random intervals.