Replacing toilet supply line valve.....

I'm replacing a toilet supply line valve.
Toilet is on lowest level of house, and supply line valve is literally the lowest valve in the house and supply line is literally the lowest water line in the house.
I plan to turn off the main shut off, and open cold water valves up on third floor (the upper level) and the cold water valve in that same lowest level bathroom's sink.
Then I plan to open the toilet supply line valve and drain that line into a low flat pan before removing and replacing the old valve.
Is there anything obvious (or maybe more importantly, not so obvious) that I'm missing regarding draining that line?
I *think* I've got the drainage issues / air issues / vacuum issues covered, but I'd rather avoid draining 10 gallons of water into a pan that holds maybe 2 gallons.
I'll have the wet dry shop vac and a bunch of towels handy in any event.
Thanks for your comments.
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I replaced my toilet supply valves a few years ago. I don't remember how much water drained out, but it wasn't enough to surprise me.
My advice would be to disconnect the water supply line to the toilet first, and then drain the pipes through that before removing the valve. That way if your pan fills up, you can still turn off the valve, empty the pan, and open the valve again. You'll still get a little more drainage when you remove the valve itself, but probably not much.
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Good point!
BZ wrote:

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Why not flush the toilet several times before taking the supply line off to make sure you have nearly all of the water out?
JK
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No plumbing wizard but I would think, assuming the valve somewhat works, you can use the valve to stop the draining so you can empty pans, wring out towels, call 911, etc.
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Before touching the toilet valve/supply line, open the faucets in the bathroom sink next to the toilet to lower the water level down to that point, there won't be much left below that point providing you shut off the supply to the house.

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EXT wrote:

Please read and comprehend the initial post before replying.
> I plan to turn off the main shut off, and open cold water valves up on > third floor (the upper level) and the cold water valve in that same > lowest level bathroom's sink.
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