Like Jerry said, the nuts crimps on the line when tightened and may reduce the OD of the line in the critical point where it's supposed to seal against the fitting "wedge". If they whole deal is installed correctly, with a little bit of tubing sticking out past the nose of the nut, it's usually not possible to get the nut off the tubing without destroying something. The tubing with nut crimped on it can "sometimes" be reused and made to seal up, but as Jerry said, typically it's best to replace the nut and tubing together. If your tubing is still the correct OD, you may be able to get away with just replacing the nut.

If you look at your photo of the line coming up thru the head, you can see a slight ring just below the end, which is where the nut has previously compressed it, reducing the OD there. It may not reseal even with a new nut.

Last edited by klhansen; 11/17/2017 12:24 AM.
Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.