Fuel tanks acting strange, '75 300d

[sailorbenjamin]

Two questions. Got a 300d fresh out of long term storage. Won't start. Got bubbles in the fuel lines and I don't know where from. Decided to replace all the rubber fuel lines to start.
Easy question first. Two hoses coming out of the tank, right? Upper is just straight 5/16. Lower looks to be 3/8" at the line and 1/2" at the tank. Am I seeing this right? Would MB actually do such a thing to it's loyal followers? Where do I get a replacement?
Hard question. I remove this lower hose and not much comes out. Just a few drops. Then I remove the upper hose and a bunch comes out of the lower hose, like I've released a vacuum. So I'm assuming the lower hose is the feed hose and the upper is the return. Does that make sense? And I'm concerned that if nothing comes out of it then maybe I've got a plugged vent somewhere that's causing the lack of fuel pressure at the pump. What do you all think?