I have time to quickly make a "cut away view" using MS Paint & other programs.
This is if you cut a connector long ways to see them as installed on a car.



Blue is the "nut" on the gas filter etc.
Yellowish is "Captive Nut" on the metal line.
Just imagine the threads where Blue and Yellow actually hits each other.

The "Flare" on the Gray line Does Not Seal the joint like most other Flare Joint for Brake lines & other Plumbing.
All loads there is clamping force on the Gray Metal Line & to stop vibration hitting pulling loads like when rubber sections is pulled from engine torque.

Only the Viton O-ring seal the joint when slightly compress when end on line slips into the smooth hole walls. (& why I drawn the O-ring as oval shape.) The Nut(s) Do Not put any other pressure on that seal.

If you have damage in the Blue nut &or on the Gray Line then the O-ring can Fail to Seal and Nothing you can do to seal again. Worse Teflon Tape and others Fail & parts of them plugs Fuel Regulator etc.
If you're not careful then often Damage the O-ring hitting the treads etc & won't seal even for new rings.

Note that Viton O-rings are often Tan to Orange or other Colors including Black. Do Not use "Standard" Black O-rings to replace them or expect to leak & very soon since most areas gas has Ethanol.

Can I Reused the O-rings?
⚠️ Should Not but most will try because often see the "ripoff" price by Dorman and other to get new ones. (PN 800-013. 4 in 2 sizes for $5? Most think No and won't look elsewhere.)
In Low Pressure Systems like Duke TBI using 9-13psi... if the used O-ring doesn't leak now is likely "ok" and never leak.
Pump side of High Pressure systems is more likely to leak w/ used O-ring.

Return Lines have near Zero pressure & easy to seal w/ used O-ring.

Notes:
● Many Viton "Kits" from HF etc may not have the sizes GM uses. Check careful & fast before Return Window Closes @ HF Amacon etc. May not be able to return this w/ some vendors.
● Also TBI & Carb "Rebuild" kits for GM have 2 Viton O-rings. Even if you don't use them, don't lose them either.
● Dorman etc makes replacement lines w/ this end setup. www.dormanproducts.com 55180, 800-170, 800-156 and others

⚠️ Ignore whatever IA post here.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 09-05-2023).]