Daul tank fuel line plumbing

[pancake]

I recently insatlled a second tank in my 1983 GMC 1-ton (K3500) single cab long bed 4x4 with a 496 ci engine. The engine has a carb...not FI.

I installed a switching valve in the stock location under the passenger side cab and a selector switch in the dash. The valve and the switch both work fine.

The sending units on both tanks have two hose fittings on them. I attached the supply line of each sending unit to the swithcing valve and each fuel gauge wire from the sending unit to the switching valve.

My question is what do I do with the second hose from each sending unit?

There are plugs on the switching valve so I can't plug them in there.

Are these samller hoses from the sending unit a breather hose? Can I plug them and put a vented gas cap on?

Should I put some kind of breather cap/filter on them. Wiil gas overflow from the tank out of these lines?

On the orignal tank the samller line is pluged with a bolt. When I open the gas cap a bunch of air comes out. I dont think it is vented.

Any help would be great.

Thanks.