How did you pressurize the system? I had the smallest of hairline cracks on my F3's rad. Was only able to find it when I took the rad to an industrial rad service shop that works on big truck systems. The were able to spot the tiny tiny crack and fix it. Also, you prob checked this but is the washer on the nut you pictured above in good shape? When I changed mine, the nut was rusted to hell and the washer was almost completely destroyed. Ordered one from Startwin. Good luck with the repair.So all that being said. I'm still at a loss where the coolant is coming from lol. I've pressurized the system and sprayed leak detector soap all over it and nothing. I may drain the coolant next and try it with strictly just air in the system to see if i can locate a leak.