So, stopped for gas for my '73 F100 and I could hardly get the gas cap off.. Nothing appears wrong with it.
Then, gas would not go into the tank without it blowing back out of the filler tube.
I took off the vent hose and blew air into the tank and out of the hose. No obstruction.
I finally got the tank full with on and off with the gas pump.
Earlier this year I noticed a little of this issue developing but this time it was a real difficult fill.

 

A few questions:


Was truck running when you were filling up? (I know the right answer LOL)
Gas cap hard to get off due to? Over pressure or vacuum?
Look at your filler hose and vent hose and make sure its not crimped going over the frame rail.

 

Truck wasn't running. The vent hose was pretty stout and round. I had to cut it to get it off. Gas tank has never been off.
Why is it always the tank is full when it has to be removed? LOL

 

The only way it could be spitting back at you is an obstruction in the fill hose or somehow gas is getting redirected into the vent hose. 77 and lower require a 2 way vented cap which isn't made anymore so drill a 1/8' hole in the cap or at the top of the vent tube. I have seen some gas tanks with so much vacuum that it has collapsed the rubber portion of the fill tub.

 

If the filler hose is flattened where it goes over the frame, replacement filler neck hose is available at NAPA, but it is not cheap. They sell it in 3-foot sections at 20 -25 bucks a foot. You can get it from rockauto ,but you have to pay shipping. I think the piece I bought is 2 1/4" Measure the neck of the tank to be sure. As far as the gas cap, old style pressure/vacuum vented caps are still available. Autozone lists one, part# 6040. It fits mid 60's Ford cars. You will have to try it to see if it will fit your filler neck. I've got a similar one on my 78,but im not sure what it came off of. You do not have to remove the tank to change the filler hose.

 

Bronco Graveyard carries the filler hose. It's made by Dennnis Carpenter and has the u-shaped piece of metal to flatten and protect the hose where it goes over the frame rail.

Bronco Graveyard also has gas caps for the various years.

I put a new hose and caps on my 78, and immediately started having pressure problems because the caps didn't vent. Left me stranded once, and may have contributed to the failure of my electric fuel pump. I drilled 1/8" holes in the caps and haven't had any problems since.

Your problem is likely not the caps, but possibly a collapsed filler hose

 

You do have to drop the tank to replace the filler hose. The hose snakes over the frame. What a poor design.

 

Thanks for all the suggestions. You do have to drop the tank to replace the filler hose. The hose snakes over the frame. What a poor design. The filler hose was nothing special to the eye. It was 44 yrs old and mushey. Looked pretty flat. Anyhow, full tank, siphoned the gas, dropped the tank, cleaned the corrosion, primed, painted, replaced all of the hoses, cleaned the fuel gage ground, drilled 1/8" hole in the vent, refilled and tested. Not bad for two 70+ Ford fans.
One bonus I did not expect. I have had a slight engine stumble at start from stop. This is completely gone. Totally. Only thing I can point to is the gas tank pressurization or lack there of. Pretty strange but good.

 

good job getting that tank out and serviced! and congratulations on getting that stumble fixed at the same time

 

Good deal!!!!