my 'new' used car had no visible radiator coolant, so drained, from the 2 inch hose into water pump, then filled and drove with filtered rainwater only, then re-drained and added 5 liters of red concentrate, ran engine with cap off, to remove air bubbles.
1st problem was leaking 2" hose, fitted with new worm drive hose clamp, seemingly tight.
The previous spring type hose clamp stopped that leak, but then red coolant was dripping, every ~6 seconds, from the male fitting into the plastic tank
.
The cap, CPC brand, 18 psi, looked OK, but I replaced it for now with an old '16 pound' cap from my Jeep, and the system now seems 'stable' with no more leaks seen, yet.

Should I try to tighten that fitting, or just get a new 16 psi cap?
Thanks for your advice.

Yesterday, I used a 32 millimeter open end wrench, using the wrench tips only to grip the 32 mm blue crusted fitting, which tightened slightly, with gentle pressure from the wrench. I didn't notice whether the 1/2 inch male flitting threaded into the blue crusted fitting loosened during my tightening attempt.
Is the 32 mm brass fitting simply threaded into the radiator plastic, or does it use any thread sealant(like liquid teflon, or teflon tape)?
A red-crusted washer also seems to be between the 32 brass fitting and the radiator plastic.
http://superfly.co.nz/radiator_plumbing.jpg