Sending unit # of ports
 

I have a 77 C10 which was equipped with the Big 10/F44 "heavy duty" emissions exemption. These had no EVAP or charcoal canister system, so I assume they came with a vented fuel cap?

Also, regarding the correct fuel sending unit on this type of emissions setup. Would the correct sending unit would be one with only one port? I know for a fact the truck did not come with a return line coming back to the tank from the fuel pump.

There are about 3-4 pages worth of fuel sending units in the parts breakdown, and i've seen 1, 2, 3, and heard of even 4 port? sending units so it's a bit confusing.

Re: Sending unit # of ports
 

My 75 is the same. Non vented gas cap. Vent was done via the sending unit. You want a 2 port sender. One 3/8" goes to the frame(or tank selector for dual setups) and the other one has a very short hose that ends in a plastic capped vent.
Vent is similar to this: https://www.amazon.com/Inline-Tube-R...8269286&sr=8-5

Get a vent that matches the 2nd port on the sender.

Re: Sending unit # of ports
 

Ok that makes more sense. Rock Auto was giving the option of a 2 or 3 port sender. I hadn't been able to get up above the tank to see what the stock setup looks like, so was just assuming the venting was done on non-EVAP trucks through the cap.