I had at least a half-dozen HVAC guys tell me that my baseboard
radiators had no bleeder valves and that the only way to get air out
of the system was to drain and refill it. Even did it a couple of
times, with me standing there saying "But the water you're adding has
air in it too. How's this gonna be any better?"
Finally one guy pointed to a tiny thing near the boiler, said "That's
an automatic bleeder, oh, someone screwed it closed." He opened it a
bit, after the air was bled a bit of water came out, he said "That's
probably why someone closed it, I'll replace it" and now, no problems.