Thank you very much. That is it !
- The cap is definitely on the vapor line near the solenoid. Yes, it is very similar to capped A/C schrader valves (I didn't want to touch anything at this point).
- Yes, following the line got fuzzy as it went down, and being integral to the brake lines made no sense to me at all.
- I have had misfires for a long time, and have tried many things. When I replace the o-ring on the valve, and the misfires stop. (Since the o-ring (Viton) hasn't been fitting very well, it gets squished out, and a leak must happen shortly after.) The leak here seems to be one of several that, in culmination, have sealed up vacuum leak problems. I now believe that vacuum leaks are a very big culprit to misfires that are easily glossed over (I know I did).
Thankyou again!