If the pipes are below water level . . . yes, it will drain, if you open them.
If you send pictures of your piping and valves, we may be able to suggest some ways to work on them 'wet'. Two possibilities are a Cherne blow-up plug, extracted through a Valterrra slide valve, once the valve is in place; OR if you have threads to work with, simply working fast, and screwing the appropriate size ball valve onto the threads several turns, before closing the valve.
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