So on my 1974 Duster I can only put MAX 7 gallons of gas into it. It has a stock style gas tank like my 73.
The gas gauge works normal, so from just under a quarter take it about 6 gal. and then gas gauge reads full.
~ I dont know why I cant put a full take of gas in it tho?? It has been like this since I bought it.

The gas cap does not ever make a pressure release noise at all when I open it.

I found this black canister under the hood and tried figuring out what it does. There is a rubber hose connected to a metal line that goes under the car. This hose is not connected to anything. Is this part of the fuel system? If so will it fix my problem?

Below are pictures of this canister with 3 ports

 

That 1/4" line is your return/vapor line. If it is clogged then it will be hard to fill up the gas tank. If you can blow it out from the canister end and make sure it is not clogged.

 

@bremereric, Thanks What are the 2 other ports for then? Does something need to be hooked up to make this work properly?

 

The other two ports used to connect to the carb and such. None is needed anymore.

 

I hate to be the bearer of bad news, but that vent LINE has nothing to do with fuel filling. The venting filling is completely dependent on fuel "running" down the bottom of the filler tube, and air pushing back out the top. There's been guys with early "A's" as well as 67/ later which have the same vent at the top of the filler.

If you are in a state with the booted fuel filler nozzles, this is a factor

One thing which MAY be a factor is the construction of your tank. I believe you have the later setup where there is no separate vapor separator? That is, you do "not" have a separator tank up above the fuel tank?

The diagrams in this thread are showing that in 74, the vapor separator was built into the inside top of the tank

It might be that this stuff is all gummed up in the top of the tank

Here are some diagrams over the years

The 74 should have the vapor separator

The other thing is, you may have a damaged fuel pickup, either bent from physical damage, or rusted through, etc, so that it does not pull all the fuel from the tank. Therefore when you try and fill the tank, it's already got some fuel in it. Same deal with the fuel gauge. After all, these cars are getting a little old, now.

 

440 is right on. The canister you are showing is the purge canister and pulls vapor from the tank after it is caped off. Not when fueling the car the only way it even works is if there is a vacuum line that goes to the purge cani. and the purge valve opens pulling vapors from the gas tank and burning it in the engine. On the canister it even tells you purge is from the purge valve or vacuum line, tank is the hose you have in your hand, and the other one is carb bowl vent which goes to the bowl vent on a stock carb. If you dont have a stock carb disregard all of thatand put the fuel tank line back on and call it a day. Note: there is a filter on the bottom of the purge canister you may want to change its made of fiberglass