Am I reading this right, you have two rolled up bits of absorbent pad, the one on top is dry, the one on the bottom (at the bottom of the valve body) is soaked with red diesel?
My FIRST ANSWER WAS GOING TO BE: I think your answer is disassemble, reseal. Its very likely there is a close pipe nipple between the manifold and your valve. Start with a fresh one, so the threads are worn/mashed down, and use a good sealant. I use rectorseal #5 for all fuel/oil connections where a sealant is indicated, but I'm sure there are lots more. Not sure what is on yours, reactorseal is a pale yellow color.
My SECOND ANSWER, after rereading your post - is that you need to disassemble and inspect the manifold/manifold threads. Delrin is an engineering thermoplastic, and you have metal going into it. Delrin is the trade name for DuPont acetal resins. It appears from how the fitting on the side is made that you have machined thermoplastic female pipe threads in the manifold. Brass and stainless steel have a different rate of expansion in changing temperatures, and over time will mash the threads and start to leak.
In looking at DuPont's website, the expansion co-efficient for Delrin (depending on resins) ranges from 10 x (10^ -5) to 14 x (10^ -5). Its hard to write this in TrawlerForum - read 14 times ten to the negative 5th.
http://www2.dupont.com/Plastics/en_US/assets/downloads/design/DELDGe.pdf
Steel thermal expansion co-efficient is 13 x (10^ -6), an order less than Delrin.
Thermal Expansion Coefficients
For comparison, polyethylene, used for most water/waste rotomolded tanks has a TCE of 18-20 x (10^ -5), almost double of Delrin. We know not to use brass fittings in those tanks because eventually leaks are almost automatic.
If you convert all the above thermal expansions into the same units (10^ -6) they look like this:
polyethylene 200
Delrin 100-140
brass 19
steel 13
This means to me as your fuel and/or engine room changes temperature from 40 F in winter up to 75 or 80 in summer and when the engine space is warm you are getting the Delrin expanding around 10 times more than the metal fittings going into it. While this is a tiny amount, its no bueno to seal fuel. Depending how much fuel is circulated and warmed by your engines, you might have that thermal cycle every time you run the boat...
My worry is either a stress crack from machining or the threads are wearing from the above. And just resealing that one might mean the others are on the way to the same condition.
IF I'm on the right track, and that's a mighty big IF, the solution might be to switch out the plastic manifold for a metal one.