Here is a call we get in a house like this. The homeowner says the main floor is warm but the back bedroom upstairs never gets past lukewarm. Quick observation first: we confirm the thermostat is calling, then head down to the boiler. The gauge reads near zero on a 0 to 30 scale, which tells us almost nothing, so we look at the pressuretrol settings instead.
This one was set to cut out at 5 psi. That is far more than a one pipe house needs, and the steamfitters who piped these Tudors heated them on ounces. We turn it down so the boiler cuts in around half a pound and shuts off under 2. Then we pull the pigtail, because a pressuretrol set right still lies to you if sludge plugs that loop.
Next we follow the main to the end. In a house like this it often runs along one basement wall and splits toward the back. We find one main vent, and it turns out to be a small radiator vent somebody screwed in years ago. It cannot move enough air for that length of pipe, so the back of the house fills with steam last, if at all.
Upstairs, the bedroom radiator vent is painted over and the radiator pitches away from its valve. We shim the far legs so condensate drains back, replace the vent, and leave the supply valve fully open. With a proper main vent added downstairs, the bedroom heats along with the rest of the house. We write the new settings on a tag at the boiler so the next tech sees them.