Where can I find an online diagram for the air lines on my truck? I have an '88 F150 with a 351 cu eng., my pcv valve has an upper and lower fitting for air lines. The upper fitting is missing a line & air is leaking. Can't seem to find where the air line should be.

 

I think you mean vacuum lines and a vacuum leak.
On some engine applications, the PCV system is connected with the evaporative emission system.



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On my 88 F150 with a 300 six the PCV valve has a hose from the intake manifold to the PCV valve. The valve part plugs into the valve cover.

Aftermarket PCV valves are made to be used for more than one application.
They usually have plugs you can insert into a unused ports. Here's a picture of a PCV valve for a 88 F150 with a 351. The valve has two ports. I've circled in red the two dummy plugs that you can break off and use to plug any unused port.

 

That is the same PCV valve that I have on my truck. What I was trying to say was that the upper port is open and has no vacuum line attached. I wanted to know is that port supposed to be blocked off? or is there a vacuum line somewhere that I don't see?

 

Here's a picture from the 1988 Factory Service manual. It shows the PCV valve for the 302 and 351. It shows only one hose that goes from the valve going to the intake manifold.

So you need to plug the 2nd unused port on the PCV valve.