[Red Mustang Gt 46]

I'm sure youv'e probably already tried this but... before you insert the tool (I assume the "duck bill" type), press the fuel line into the fitting and then insert the tool until you hear a click, this relieves any pressure between the fitting and fuel line...you can also turn the tool on a clock wise motoin to make sure the fitting dis-engages on all contact points.

Also, are you using the right size line lock release tool, 3/8 or 5/16th?