Venting a low return in the spaghetti factory.

Hey everyone. Up front, 1890s commercial property, 2 pipe steam. 2 condensate return tanks remote from the boiler room, 1 in the boiler room and a boiler feed tank. There are 2 boilers. One is abandoned in place and sits in a pit about 3 feet below the floor of the current boiler. For a time, it was maintained as a backup meaning condensate could be routed to either. This resulted in some unorthodox return plumbing that I refer to as the spaghetti factory and this sets the stage for my current situation.

There are a handful of rads on the boiler level of the building. They are served by two 3/4" returns that disappear into the concrete floor and emerge in the boiler room as a single 3/4" copper pipe (I know). It's the ability of this return to effectively vent air that I'm concerned with.

Visual aids incoming.

So as we can see, anything (liquid or gas) needing to exit that copper pipe must pass through a permanent water seal. Will this prevent or seriously slow the venting of the rads that use this return? Being a former coal burner with no mains venting, this whole system is already slow to heat.

If I were to simply sweat in a Tee (in place of that compression coupling perhaps) and extend a few feet of pipe up, left open to the air, would it make much of a difference? I'd turn down the top of the pipe to keep anything from falling in.

Thoughts?