Hi everyone. Remodeling lower room in home had access to heating pipes so updated to rooms to slant fin baseboards from cast iron. Decided to change 20yr working old circulator pump, drain valves, and leaking hi- vent at burner. Put heat on bled every baseboard, drain , etc. Now have heat at all baseboards on both floors except the main floor two Baseboard heaters that are the farthest from the rest. If I bleed either one, the baseboard gets hotter and works perfectly. Then after a very short time, baseboard gets cold again. What am I missing? Thankyou for your time everyone. Gene