Air keeps returning - ideas?

I bought a 1954 home in May. Moving into winter, I doubt the woman who lived here was EVER warm enough. The first days below freezing the house wouldn't get over 67F. I work on the industrial side of the business but I am not experienced with the commercial products used in this house but am learning quickly. First thing I started was bleeding the air out the system. The water was so black that turned into shut down and flush. Before starting, I noticed she had a plastic pan under the pressure relief and there was a small amount of water in it. So while I had the system down and cold, I relieved the pressure and checked the pressure on the surge tank bladder. It was at 30 psi. Since this is a single story home - boiler in the basement - I dropped it to 15 psi. I set the make up regulator to give about 18 psi cold and 25 hot (there is some some creep which I think is normal). Bled the system of air both at the boiler then at the baseboard radiators. The pressure relief stopped it's intermittent spitting...for a while. The pressure crept back up which tells me the make up valve is sticking or leaking. It had warmed up outside for a while so I stopped working on it. When it got cold outside again, the system still couldn't keep up. I went and checked the temperature setting on the boiler and it was set for 160 F! It wasn't that cold outside yet and all I could think of was the poor woman freezing her but off last winter. Set the temp up to about 185F (Per weil mclain manual) and that problem went away.

Air came back again - sounding like a drain pipe when a toilet flushes in a couple of rooms. Replaced the two TACO vent valves with Watts vent valves and bled the system again at the radiators - twice now. I’ve kept an eye on the pressure and shut off the make up after bleeding so the pressure would stay the same. Check it every couple of days to be sure it doesn’t drop until I replace the make up valve. Lasts about a week and the air comes back. No leaks anywhere that I can find. I've purchase a new regulator make up valve with a vacuum breaker because a county inspector noticed there wasn't a vacuum breaker(don’t get me started about dynamic water pressure being too low here – ok for boiler but not for water powered sump pump). Also bought a spirovent jr. to install though it will will only fit in the main supply leg to the original radiators and not a small loop added later when a breezeway was closed in and more radiators added.

I beginning to think my issue is that the water boils in the heat exchanger because the flow isn't high enough. I can hear it very slightly just before the burner shuts off. Of course I may be hearing air or just expansion from heating up, it’s not continuous. Before I cut in to replace the make up valve and install the spirovent, any suggestions? I am used to having supply and return gauges with estimated pressure drop so I can tell if the differential pressure and flow are correct on the big industrial jobs I work with. Being an older house with no info on the design, I have no way of telling what the correct differential should be. How can I check flow? It looks like a Grunfos pump. Will amperage on the motor be accurate enough to tell me if the flow is in range? One big circuit - no zone valves or zone circulators.

So suggestions welcome before I replace the make up valve and add a spirovent.