There is a very small tube with a 90 degree barbed elbow on the end of it, hanging loose.
It is located under the power assist canister and the other end connects to a sort of manifold at top of firewall. That is, there is a tube going across truck at top of firewall with some small tee fittings.
I cannot see where it might plug in.
If that is not it, what is a description/ location of the vacuum line, you mentioned?
What spring are you talking about, where is it?