Any info out there on a Trane 2 pipe convector system?

I have not checked all of the connectors but I have checked a few of them. None of them have supply valves or traps. I also don't see any traps in the basement.

The ends of the two mains have Hoffman 75 vents.

The ends of the two dry returns also have Hoffman 75 vents.

When the system runs, it hammers like crazy everywhere. Water spews out of one of the Hoffman's at the end of the dry return. The other vent on the dry return is failed closed.

Since the pigtail looked like it hasn't been touched since the boiler was installed I checked to see if maybe it was plugged. Sure enough it was plugged.

The pigtail was the old iron type, extremely difficult to clean so I replaced it with brass. When I replaced the pigtail, I told the owner I would like to try a vaporstat to see how the system operates at a lower pressure. I raised the temperature 4drg above room temperature. All of the convectors were heated and there was virtually no hammering and no water spewing out of the dry return vent.

I still need to do a radiation survey to see how well matched the boiler is to the installed radiation but is there something else I should be looking for?