I drained my sons heating system last Friday and installed 2 new radiators and capped off another after removing it. We used a combination of speedfits and had to solder 2 elbows. Anyway the system was refilled and worked fine until yesterday when the pressure dropped to zero. He re pressured it last night but it dropped to zero again this evening. He’s bled the system and says there was plenty of air in it but no visible leaks where we’ve been working. Is trapped air likely to be the cause of this?