1995 for e150 cant put gas in tank

have a 95 e150 conversion van which will not let me put gas it.  the pump keeps tripping off.  i have been able to very slowly add gas with a gas can.  when i did i heard a clug clug sound as air was trying to escape to allow gas to flow into the tank.   i checked the rear of the tank and found a round plug that had 2 tubes coming off it one soft that ended in about 10 inches with a plastic cap that was removable, the other was a hard line and move along the rail to the front. since there was no dirt or spider nest in the soft plastic tube due to the cap i am thinking the vap canister is the problem. have not been able to locate it. if i do can what can i do to test the theory and allow gas to flow prior to replacing the canister.