Show a picture of the lines feeding the section you would like to remove. The confusion is coming from you saying you had a loop system. Once I saw that you had cast-iron I realized that couldn't correct. You can not purge a one pipe system. It must be vented at each section of baseboard. The procedure for draining will be the same.
Here's a video I took of it, hopefully this is better than picture.
https://www.flickr.com/photos/mranvil/23954955585/in/datetaken/