I've replaced the return line washers and the small banjo bolts on the injectors are as clean and tight as they can be. There's still seepage around at least 3 of the injectors. The high-pressure lines/nuts are dry.

What could be wrong?

 

When I had that issue, it was the big hold down nuts weren't tight enough...and it was coming out of the bores. I think...lol.

Also, possible valve cover gasket issue?

 

I have had issues before. What I did was get them tight, then lightly tap on the banjo and retorque. Just be careful, they are pretty weak. I was afraid also of stripping the threads.

 

Have you changed the injectors lately?

If yes, did you reuse the old copper washers or install new washers?

I sometimes reuse the washers.

I sometimes have leaks.

 

just torque down the hold down nut to 44 ft lbs. it should stop the leaky injector