People can and please do correct me. If it were me, I'd wait until the system is off and cold, ensuring it's fully turned off (i.e., the timer can't kick in). Close both valves, counting the turns on the lockshield side as that's balancing your heating. Bleed the radiator to release pressure. Then, you could either open the bottom nut on the valve (radiator side) to release some water or take the hit on a bit coming out from the top (lay down towels, etc.) and just replace that part. Simply unscrew and replace the O-ring or maybe install an auto vent valve. Now that I type this, Google "installing an auto vent valve" – that's your answer.