help please! fass/tank leak somewhere

[01BIGRED]

i just put a new fass on my truck and somewhere at the top of tank i am loseing fuel. we can not find the leak anywhere. we have droped the tank checked the cap checked all the lines and still no signs. but the weird part is it only leaks while driving so it makes it a guessing game. any help is wanted. thank you

 

[Jff24Gordn]

Pressurize the tank with a air hose.

 

[Diesel-Dan]

i just put a new fass on my truck and somewhere at the top of tank i am loseing fuel. we can not find the leak anywhere. we have droped the tank checked the cap checked all the lines and still no signs. but the weird part is it only leaks while driving so it makes it a guessing game. any help is wanted. thank you

2 things:

Is your return line from the engine tight?

Did you have an intank pump?

One other thing you might be seeing is residual fuel from your stock filter head and feed line draining back. This is normal to some extent. BUT can be worse if you didn’t drain your filter head. Might try that, clean the tank, and see if the leak goes away. I bet this is your issue.