I have a 2011 XLT 5,0 Supercab. Recently I attempted to clean out the HVAC drain, which is located behind the right cylinder head on the 5.0. During this attempt my arm hit against the vacuum lines going to the right hub. This line broke like a dried out twig! Upon further investigation I found that the entire line was junk with the exception in the hoses at the hubs. This line is 1/8" plastic which does not age well.

I found on E-bay an OEM replacement for under $50. The factory part number is BL3Z7A85A. Replacing this in my garage took about 2 hours. I would recommend if you do this to start at the front cross member and work your way out to the hubs. Do not start at one wheel and try to work across to the other. It's NOT fun.

If you have this line break, the hubs fail engaged. Then you'll have a small vacuum leak where ever the break is.

If you have an older truck with the vacuum hubs, this might be something to check.