Hmm. I’ve never drained a water box unless I was parking a machine for long term storage. I don’t see any reason to do it when the machine is used daily. Just be sure to keep the machine from freezing in the winter.
I've never done that and I can't see any reason why that would be in the manual. The only time you are supposed to drain the water box is when you're doing flood work. It might not be a bad idea to drain it if the machine isn't being used daily or at least several times a week. Another reason to drain might be if you're winterizing the machine. I've never had to do that since mine's garage kept. Just keeping water in the water box does no harm whatsoever under normal circumstances. I haven't done that on my Boxxer 421 in over 16 years. Which Hydramaster machine are you talking about?