In 1897, Eureka made an early version of what we now call the diverter tee. It worked by creating a drop in pressure along its run, which encouraged the water to flow into the radiator. Here's a picture of one found on a job. The contractor who sent this kept his hands in his pockets until he learned what it was. Smart man.
You can view the patent for the Eureka here.
It pays to poke around a lot before you start taking things apart on these old systems. If you're not sure what you're dealing with, take a photo of it and post it on The Wall at HeatingHelp.com.