new tank leak

[bobmac25]

Hi all, I recently installed a mustang tank in the frame of my 69 lwb. After install was done everything was fine. about 3 days later I noticed it was leaking at the sending unit. I removed the fill cap and released pressure the leak stopped until the pressure built in the tank again. I have already changed the gasket 3 times but no luck. Does anyone know of anykind of sealant that can be used to stop the leak or perhaps a new cap that will keep the pressure from building in the tank.. Thanks.

[my5thmopar]

If you are releasing pressure then something may not be vented correctly. I'm not quite sure but, if using a newer tank, do you need a different cap. I don't think early Mustangs were vented but, I don't know. What filler neck and cap are you using? Most of the time you have to use a hammer and block of wood to tighten the lock ring. It shouldn't be easy to put the lock ring on so, the pressure is leaking at the weakest point.

[bobmac25]

The filler neck is the orig. from the truck I had to cut it down to fit, same cap. well a new one that fits the neck, old one got damaged in taking the bed off one of the times. Yeah the locking ring was not easy to get on but I made sure each time it was in the lock groove.

[PwrWgnDrvr]

Why are u "building pressure"?
Tanks do NOT build pressure, unless vented when cold, then heated during the day, and only when using a non vented cap.
A vacuum is created as the fuel is consumed and unless "vented" to allow air in, they will collapse from atmospheric pressure on the outside.

[bobmac25]

That's what I'm not sure about. The only thing that changed was moving the tank out of the cab to under the bed. I'm using the same cap as before, but now when I take the cap off the pressure almost pops the cap off when I'm removing it. The build up usually occurs during the day. During the evening there is no pressure build up. I didn't have this problem with the old tank with the same cap. I'm at a loss. Is there a cap that will not let the pressure build?

[Txas2step]

I'm pretty sure you need a vented cap. Your old tank in the truck must have a vent line.