When I undid the solenoid, only the top half came off. I do not know if that is correct? It might be because it came apart pretty clean?
Nope, definitely not correct. Some of the replacements do have a seam there, but it was crimped together originally. A poor design, in my opinion as the plunger tries to drive that joint apart every time you engage the solenoid.
Anyways I do want to drain the OD. Would I pull the large brass plug of the bottom? And if so, are there some suggestions for a tool to loosen it?
The usual suspects offer purpose-made tools, or there are several approaches to building your own. Here are my two versions:
The one in the background was my first attempt, made with common hand tools and material from the local hardware store. Home Depot has the big flat iron bar back in the building materials section, where it's sold for a concrete form stake. I drilled and tapped two holes, inserted bolts, then filed the bolts to match the slots in the plug. The result worked well enough, I used it for many years, but was a bit awkward to maneuver under the car. So after I got the mill, I made the other one from a big socket I found on eBay for cheap. I think it's an 4x4 axle adjusting tool or something like that.