Is that the one in the hose that goes to or comes from the interior heater? I have never needed to touch mine, just been patient in bleeding by leaving the coolant reservoir tank cap off and running the engine up to temperature...
Maybe they do go "off" with age? If the screw comes right out maybe you can try to put some PTFE tape on the threads? But you'll need to partially drain the coolant again to avoid making too much of a mess.
2000 Pearl White Plus & 1996 Portofino 20vt
1985 & 2016 2017 Fiat 124 Spider + XF Sportbrake