If what you are doing fails to solve the problem go down to the boiler with a 5 gal. bucket with handle!!!
As you are facing the boiler the pipes on your right are the pipes that supply the heat to your base board !!!
As you are facing the boiler the pipes to your left are the pipes that feed back to your boiler!!! One zone =one thermostate!! two zones =two thermostates etc:
the pipes to your left should have on /off faucets handles like your out door faucet or lever!!!
Take your bucket and hold it under faucet on left side of furnace, open faucet just enough to let water run or maybe a little more [you be the judge] when water is not cloudy shut off!!! If two zone there should be another pipe, do the same!!! If three zones etc: repeat as you did the 1st zone!!!
Furnaces have automatic refill!!! It will fill on its own!!!
P.S. Do not turn faucets on like you would a outdoor faucet!!!