Are fuel caps supposed to vent?

just to get the ball rolling the later units like my o6 have a vent to atmosphere that holds
until a certain point (psi and vacuum )
not sure on yours but if you were having fuel delivery issues i could see a failed vent (or maybe cap) being your problem
hopefully someone with your year can clarify if the cap or separate vent is used
but maybe elaborate on the issue you were chasing ??

Only on the interstate, and only above 65mph, it will begin to starve for fuel. After switching to the front tank for a couple of minutes, I can switch back and everything is fine for another 5 miles or so. Number of miles before it builds a vacuum varies. I think it depends on how much oil has splashed up on the cap.

If a new cap will act exactly like this one, I'll just drill a small hole in it and make sure it's pointing down all the time.

When mine did that, the problem was the shower heads broke off.
Changed the fuel pump twice before I even checked in the tank.
Ended up having to blow out the lines and selector valve.

The front tank doesn't have a shower head. I haven't dropped the rear. Today the problem went away when I loosened up the cap. You can also hear the thing suck in a rush of air if you take the cap off while the tank's low. I'm going to run with a loose cap or a couple of days and see what that does.

That's how mine started. I swapped the caps around. Drove another 20 miles to a museum place. Went to leave and only made it a mile.
Towed it to a buddies house and replaced the fuel pump.
Few days later starts acting up again, Warranty the fuel pump and swap it again.
Barely made it home.
Then I remembered to check the pickups, Both were gone. I took a light and reached my arm into the tanks and got everything out, and extended the pickups, and blew out the lines.
I'm still running the same caps, neither has had that vacuum since.

IIRC there's a vent on the top of the tank; maybe it's clogged?

I'll keep that in mind. Today it's been fine, but most of my driving has been in the city. I'll be on the highway tomorrow, so I'll see if the loose cap works then, too. It never seems that I have a convenient day to run the rear tank dry, then pull it. If I can fix it with the cap, I'll do that for now.

Come to think of it, the rear tank probably doesn't have a shower head, either. I can't suck the last 1/4 tank out of it. I like the idea of running extensions to the bottom of the tank.

What was weird is both of mine broke off at the same time. Just a week before, I could run both down to E.
I would have to check, but I do think there is a vent on top.
And mine were easy, with my flatbed I was able to pull both senders no problem.

I don't have a flatbed, so it's not so easy. Today I was able to go about twice as far before I had the starving issue. It doesn't do it in the city, and it only does it on the interstate if I keep it above 65. I'm going to try blowing out the fuel line next since that's pretty easy to do. I wonder if it has anything to do with the viscosity of the fuel. The lines on these trucks are pretty small as it is. The WMO is quite a bit thicker than diesel. I have some cheap fuel oil that I top off with sometimes and I'm always amazed at how fast my oil pumps pump it. If I didn't pump so much oil, I'd probably be amazed at how slow oil pumps. Anyway, there's a big difference. I wonder if I'm simply not getting enough flow through the stock lines.

Could you throw a few gallons of station diesel in that tank and run it a while, just to see what it does? Depending on your location and the temperature, you may be on to something wrt. the viscosity, and you may want to run a mix for a while, if that's possible.

I plan to try something like that. I'm going to stick with one variable at a time, though. I'm going to blow the supply line out tomorrow and see what that does. I don't expect it to fix it, but I need to rule it out. After that, the next easiest thing will be to throw in about 5 gallons of fuel oil and see what happens. If that doesn't fix it, I'll drop the tank. Dropping the tank is just such a sandy, dirty, pita. I'd rather be covered in grease and oil.

I had to make an unexpected trip across town - about 30 miles each way - and the truck was being a pain. So I decided to blow the line out so I could test it on the trip. Something made a loud pop - I thought I blew the transfer valve apart. I think it was the shower head because I didn't have so much as a hiccup the entire trip.

Could've been. Now to Clean them out, so it won't happen again.