Hello all. The vacuum lines were redone on my truck, but I notice that both the lines going to the hubs are under vacuum. From my understanding one of them should be a vent. My question is the small hose or the big hose the vent? Much appreciated.

 

If you pulled them one at a time, leaving the other hooked up, and hear a hiss, normal as vent is closed.
The vent valve I not at the IWE, it's a valve at the other end of the vent hose, so vacuum will be the same in both hoses at the IWE.

 

They should only be under vacuum when you are in 2wd and the engine is running.

https://www.f150online.com/forums/ar...then-some.html

Post #3 may help.

 

Figured it out. The larger tube is Vacuum and the smaller one is the vent line. Thank you.