Ford 6.9 idi injector return line help?

I am still struggling with the fuel system on the F-250 I bought two years ago, it's extremely frustrating and I wanted to see if anyone here has any suggestions. 

The fuel return at the back of the engine started leaking, so I replaced all the return lines, which on this engine are stupid plastic caps that snap over the injectors and are held in place by the fuel line nut.  I got it all put back together and started it, and 7 are fine, but the 8th one (2nd from the rear on drivers side) is pouring fuel out from under the plastic cap.  I think it's from the hard line rather than the return as it's running rough and quickly makes a big puddle.  

I took the line and cap off and replaced the o ring and made sure the line was seated properly and tightened but that didn't change anything.  

What do I do know?  My only other thought is to get a new/used line and injector and swap them, otherwise that's the extent of my skill and it's time to put it up for sale.

I'd move that injector to a different cylinder that's more visible and see if the leak follows the injector or stays with the line before trying to replace the injector line or injector. 
 

I could try that, I'm just afraid they will both end up leaking.  It's a bit more of a project to swap an injector than a gas engine involving reamers, crush washers, etc but at least it would be cheaper than buying new parts.  

I was thinking they were threaded like the GM 6.2/6.5 diesel injectors. 

Hopefully someone with more detailed knowledge speaks up. 

I went and took one apart today at the junkyard for research purposes, they have threads at the top of the injector, then a crush washer seal at the bottom that apparently needs a special tool to clean the carbon out so it will reseal properly.  The only ones I had taken apart before that were Mercedes, where it's just threaded and the other part stays in the head.  Still no idea how to get it to not leak though.  I don't have to make a decision right away but unless I can find some additional information on what might be the problem I think I'm done trying to fix it.

Likely the plastic banjo fitting is damaged on that cylinder and needs to be replaced.

Couple options for you:  

• Get a replacement set of caps and don't ham-fist them this time.
• Get a set of brass caps when they come back in stock from this guy.
  • https://www.facebook.com/DregsDiesel/
• Get a set of billet rails.  
  • https://idiperformance.com/shop/fuel-system/billet-injector-fuel-return-rails/

After looking at it again, half of them are leaking even though I replaced them all.  I don't know if I have the wrong parts, did something wrong putting it together, if one of the injectors or lines is damaged, etc.  I don't really want to just order new lines and injectors and replace it all when it's probably fine and I'm just putting it together wrong, but I really don't know how to figure it out.

All I can offer here in advice is to make sure you use some sort of lubricant on the o-ring seals before you snap the caps into place.  I think (it has been a while) I used Vaseline petroleum jelly when I did the caps on my 7.3 IDI.  And one still seeps after doing it twice.  I smeared some inside the cap and smeared some on the injector.  I did not want to pinch the o-rings.  I bought my parts from Conestoga Diesel in Pa.