Need GT40 Vaccum line help

Yeah I had the same problem with my GT40 setup. Check all the vac lines, check the capped off vac ports, check the IAC. If its none of these check your intake gaskets. I had a break in mine that turned out to be 70% of the problem

The vacuum lines you need are 1 small line for fuel pressure regulator, 1 small line for A/C, 1 small line for EGR and another small line for the smog pump. One big line at the back goes to the vacuum tree for the power brake & A/C. Another big line connects to the PVC valve. The other big line goes out the front for the carbon canister. In a pinch, one of the small lines can connect to the spare port on the vacuum tree.

See http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg for more help.