Take the plug out and check if it is plugged you can take a wire and wiggle it in and around there and make sure it is clean .once you are satisfied it is clean you may want to flush out with water as I think you said you have a new rad .they may have cleaned the block when it was rebuilt.After you are satisfied it is clean I don’t think you would use the petcock for while anyway and all it does is make it easier to open up to drain the block. I would also be careful taking the plug out as not to damage the threads and the block.
