4 Baseboard Heaters in a row with no heat

rasco

I have an elderly cast iron Wein Mclean boiler it is I believe a Monoflo T system with 2 separate loops.  I replaced the expansion tank and introduced air into a system with good flow but pressures kept creeping up until the high pressure vent blew.  When I vent any of the 4 cold baseboard manual vents I get no air only cold water.  I have also installed a hose bib after the ball valve in the same baseboard heater that is the first on the run of cold baseboard heaters.  I tried hooking a hose up to that hose bib and I can get good flow from there until the third and of course the fourth baseboard heater where I get nothing.  My next idea would be to up the pressure going into that loop to try to force the air (unless it is frozen) out of the loop to get some flow.  I am open to any and all suggestions.  Please help it is cold.  Everything worked fine until the expansion tank and all of the air that that would have introduced into the system when it was changed (no isolation valves).