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.

[tucsonjwt]

I have an 83 C20 dual tank truck but my sending units have 3 outlets - supply, return (?) and vent. Hose sizes are 3/8", 5/16", 1/4". Vent hoses are just short hoses with vent caps if you have a 3/4 ton or bigger truck, or if you have a 1/2 ton the vent lines go to a vapor recovery canister under the hood somewhere. Here is my stock setup (I converted my vent lines to long hoses terminating at the rear bumper to eliminate gas smell in the cab.)

[pancake]

Thanks for the reply Tucsonjwt.

Do I need a return line? I have an Edelbrock carb and do not know if there are provisions for a return.

I like the idea of venting it past the bumper.

The line sizes on my sending units are 3/8 supply....and 5/16 for ??? vent or return.

I do not think the sending unit is original.

I replaced the 6.2 with the big block and replaced the original tank at the same time....not sure what the tank was out of but i'm guessing an older short bed...it is 16 gallon..

[tucsonjwt]

Is your truck 3/4 ton or 1/2 ton? Do you mean a 396 engine? 1/2 ton = vapor recovery canister, 3/4 ton = no vapor recovery canister. Your original tank should have a vent line going to the recovery canister or just a short length of hose with a vent cap. A vent is necessary to allow for no buildup of fumes in the gas tank - gas caps have not been vented in quite a few years.
I don't know which line goes where on your set up, since I have 3 outlet senders, but I would guess that you need one for a vent, since the gas cap is not vented. If you take that bolt out of the unused outlet you will likely not get a big rush of gas fumes when you remove the cap.
I think you are looking at either plumbing your tanks in a factory manner or rigging up something that will work - probably what the previous owner did.
With a vent hose and cap, you will get gas fumes but not liquid gas coming out, unless your fill your tank to overfull. The vent caps are like one way valves, though when I disassembled mine it looked like just a baffle. I can post the whole vent set up I have if you want. If you have a vapor recovery canister the fumes will go to the canister - until it gets plugged up with vapor and has to be replaced.

[pancake]

Thanks again...I did edit the first post.

My truck is a 1-ton

It has a 454 bored and stroked to a 496.

I do not need to run a stock set up but I do need to run the plumbing in a safe way (properly vented and keep the presure from building up).

If you have a digram of a stock set-up that might help.

[tucsonjwt]

Sorry,I have no diagram, but others have posted it here - you can search for it. With a one ton you don't need a vapor recovery canister. The pictures I posted shows the vent lines snaking along the inside of the frame rail, and they terminate above the spare tire with vent caps installed. Vent caps are available from Classic Parts if you need them - same as differential vent caps. I bought a 24' roll of vent line off the net for the long vent lines.

[pancake]

On the stock set up is the return 5/16" or 1/4"

It looks like your vent lines are 5/16".?

[tucsonjwt]

On my truck, the vent line is 5/16". The supply and return lines are 1/4" and 3/8", but I don't know which is supply and which is return. My guess is the smaller line is supply.