steam heat baseboard vents

bob l_2

I bought a house with steam heat. It has some one pipe radiators and some two pipe radiators.

The two pipe low baseboard radiators (cast iron?) seem to have traps, as well as, air vents. Some of these radiators are 15 feet long. Should I plug the air vents to let the traps do their job? The air vents are located directly above the shut off valve, the the traps on the opposite side.

Also can you recommend a good steam heat contractor in northern NJ?

Thank you,
Bob