restriction in a fuel line somewhere

[T-ROD]

I have a vacuum/pressure gauge tapped into the fuel line in front of the switching valve (about where the bed meets the cab) and I have noticed that when running on the rear tank I am pulling about 3-5 pounds of vacuum. It has never done anything but set right on zero for the last 5 years and now all of a sudden this has started. Could there be an ostruction in the switching valve or is there some way for the sending unit in the tank to get clogged with something that would be causing this?

[T-ROD]

Thought I would add another issue I noticed while filling the rear tank today....when I took the gas cap off, it hissed loudly, as if it were pulling are into the tank. Its 30 degrees here today so there shouldn't have been any pressure built in it from heat so it has to be pulling a vacuum. Also when I got home (after driving about 25 miles) I opened the gas cap and it again hissed. Could I be pulling a vacuum on the tank itself instead of having a restriction in a line somewhere? Anybody ever heard of this?

[JSchuricht]

My 95 does the hissing thing when I open the cap but I have never worried about it. Most likely a vent line not allowing in air so as you deplete your fuel you end up with a vacuum inside the tank. It probably makes the pump work slightly harder, if your really worried about it, drop the tank, clean the pickup and vent.

[1Dieselman]

Sounds to me like your fuel cap is not venting like it should.

[backwoodsboy]

What Dieselman said.
Always start with the cheapest/easiest solution.
It is possible that a clogged fuel pickup could create vacuum in the lines, but I find it doubtful.