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You are doing exactly what needs to be done. Sounds like you are having difficulty, however, be patient, this may take a day or two to remove ALL the air. If you are still having issues after a few days, I suspect air is getting into the system from somewhere else. Perhaps the bladder on your expansion tank has a small leak? however that air also would eventually disappear after bleeding ALL the units a few times. cranking up the heat temp in the home for a bit will get things moving faster as well.
If you are able to determine which line(s) make up the return side of the system, I would attach a hose to those drain valves and open one at a time with the system all on and the zones calling for heat (temp control up above existing room temp). Also have the fill valve for your sytem open so you continue to feed water until all the air is out. To verify that the air is out, once you think its mostly gone. Place the hose into a half full bucket of water and comtinue draining the line. If you dont see any more bubbles in the water you have released all the air from that zone. Do the same procedure with all the zones.
A purge station is typically a shut off with a hose fitting above it. There can be 1 piece purge stations where both the shut off and hose fitting are combined in one piece. Or it can be a separate valve(shutoff) with a hose type "faucet" (we call it a boiler drain) above it. They are usually at the boiler on the pipe above the circulator pump. You need to determine which one goes to your problem area. I have seen them in old systems some distance from the boiler on the heating line (pipe). You need to shut the valve attach a hose to the hose fitting then open the hose "faucet". Some all-in-one fittings close the valve and open the hose at the same time so be sure your hose is attached first if you have one of these fittings. If you have zone valves with multiple purge stations all the zone valves should be opened manually. If you have zone valves and only 1 purge station then you need to open the zone valves one at a time and fully purge a zone before opening the next valve and closing the valve you just purged. You also need to jack up the pressure while you are doing this but you need to keep the pressure under 30 psi. This is done by increasing the pressure at the feed (pressure reducing) valve. Some types have a lever that you flip up from a "horizontal" position to straight up. This is the easiest type to use. Some require that you tighten the screw at the top of the valve. You should be warned that if this valve hasn't been touched in a while disturbing it in this manner can cause it to fail in which case it will need to be replaced. This device is located on the water supply line to the boiler. Be sure to turn the power off to the boiler while you are doing this. When you hear no more bubbles coming out of the hose for several minutes you shut the hose off at the same time you return the feed valve back to it's original position. Then open the shutoff. Be sure the pressure on the boiler is between 10 & 15 psi when you are done before you turn the power back on. The boiler you have has no bearing on this process. It is the same what ever type you have. Having said all that this is not something I would recommend that you try yourself unless you feel very confident about doing it. You may have to call a plumber anyway if the feed valve fails.