I have an old Standard Re-Nu shower stem that I'm planning on removing and either repairing or replacing. While it's removed I want to flush the valve. Problem is that the valve seat is recessed into the wall a few inches, and I think that after I remove the stem and barrel that when I turn the water on again that it would gush out and into the wall cavity, as well as out into the shower. I've attached a picture of the stem in the wall, and one of what the stem will look like once I remove it. Any suggestions on how to flush the valve without making a mess? Thanks.