Fuel in vapor canister

[antiextreme]

Working on my 1973 square body.  Chevy 350 (out of a '75 according to the casting) with Q-Jet.
Truck has dual tanks.  I suspect it came with an original tank on the passenger side and the previous owner added the second to the driver's side.  Was told the driver's side tank leaked when I bought it.  I dropped the tank and found that the vent tube (that's attached to the filler neck) on the tank had a long line with a top hat style vent attached to it presumably to vent the tank.
 The vent line from the sending unit was capped off and the fuel line was teed into the stock tank line.  Each tank has a two port sending unit.  There is no fuel return line on either tank, and no return from the fuel pump. 

Noticed the tank was leaking fuel out of the O ring of the sending unit, which made me think that the system was under pressure,

I connected the vent tube from the filler neck to the tank and deleted the top hat style vent.    I unplugged the vent line from the sending unit and teed it into the vent line for the stock tank.  Replaced the fuel cap with a vented cap.  I ran the truck for a couple of days without any trouble and then I started to smell gas.  Found that the vapor canister had completely filled with fuel and was overflowing.

I've been racking my brain trying to figure out how the system could be under such pressure that fuel is now being drawn from the tank(s) all the way up to the vapor canister.  Any thoughts?
(I have since swapped the charcoal in the canister with dry material but have not hooked the system back up for fear of soaking it again.)

[Rapid Roy]

I suddenly had fuel filling the canister and leaking on ground 2 years ago. Replaced canister, no more leaking. I have a single tank.

[VileZambonie]

Sounds like you hooked the vapor line up to the return line port. This is a pick up with dual tanks correct? "square body" is not specific enough info. The sending unit you describe sounds like it's for a blazer or suburban tank. For 1973: Feed is 3/8", 5/16" is vent, 1/4" is return. The suburban and blazer have the filler vent connecting to the sending unit as a 5/8" Hose.

[antiextreme]

My apologies.  To clarify, the truck is a 73 K10 fleet side long bed.  I believe the truck originally had a single tank on the passenger side but the previous owner added another on the driver's side.  From what you say it sounds like the tank might have originally been intended for a suburban or blazer, or at least the sending unit was. 

I think you hit the nail on the head.  I plumbed the vent line to the return line. 

Since I do not have any return lines should I just cap the return line port on the sending unit and connect the filler vent to the sending unit?  How would the tank vent, out the gas cap?  I thought that was a one way valve that only lets fresh air in?  Wouldn't it just build up pressure in the tank?

Or maybe I install a 3 outlet sending unit, cap the return port and plumb the vapor line to a tee on the existing line of the original tank (which goes to the vapor canister),

Thanks for the help.

[VileZambonie]

I would suggest that you put the correct sending units in and use the correct filler pipes if they aren't there. For the tank sender vent, connect to your canister and put a suitable cap on the return port or run a return style fuel pump and run the additional line.

Which dual tank switching valve do you have?

[JohnnyPopper]

My dual tanks have dual canisters, will he need a second or is the T-thingy working?

[ehjorten]

My dual tanks have dual canisters, will he need a second or is the T-thingy working?

Johnny;

I have never seen that before. All of the dual tank square-bodies I have ever owned or seen that have had dual tanks, had only one charcoal canister. Not saying it was never done, just that I have never seen that.

[bd]

Dual canisters were installed on some vehicle models to provide adequate vapor recovery capacity when said vehicles were fitted with auxiliary tanks.  Rather than produce a larger vapor canister to satisfy the specific requirements (the elegant approach) the factory chose to double up - go figure.  Canisters cannot be "dried," because the heat required to drive off absorbed fuel vapor would physically destroy the canister.  If the vehicle requires smog inspections, look for a part number cast into the canister housing then do an Internet search.  You could probably retrofit a newer, larger capacity canister to your application if necessary and reconfigure the lines with a little research and no one would be the wiser.  To reiterate that which has already been suggested by others and avoid a recurrence of canister saturation, verify the canister lines are correctly plumbed and connected and make sure that none of the lines are plugged or restricted.  In addition, verify that any fuel return lines connecting to the tanks switch between the tanks simultaneously as either tank is selected.  If the 1/4" return line from the fuel pump doesn't plumb through the same fuel selector valve as the 3/8" supply lines from the tanks, then all of the return fuel will consistently pump into only one of the tanks, which consequently will overfill and flood the canister(s) through the vent lines.

The foregoing quote is from another thread from several years back.  The gist is that vapor canister volume was dependent on the total fuel tank capacity.  Consequently, multiple vapor canisters were common on medium-duty trucks in the 70's.  Their light-duty counterparts used multiple vapor canisters if the single canister volume was insufficient for the added fuel capacity of dual tanks, even if only slightly.