Hi,

I don't think you will want to use straight water as that is one thing you do not want to introduce into the system. Besides water boils at just above 212 degrees Fahrenheit and a little higher under pressure.
Not only does it cause rust but at low stopping speeds the water will turn into "steam" and rapidly increase in volume and pressure!
Since your brake system does not have seals designed to seal off vapor pressure I would hate to guess what will happen next with that combination of scenarios?
Trust me, the change of state of the water, will change the state of your awareness, when it comes to stopping!

If you were really, really desperate you could possibly get away with using straight antifreeze. Not the diluted type with water already! That stuff shouldn't even be on the market, IMHO!
You only want to ever buy the thick stuff, the way it came for years!

Dot 3 or Dot 4 Brake fluid is made of a glycol with a different ethylene called a "Di" ethylene with an added molecule or so? I'm not a chemist!
Both ethylene's absorb water and have higher boiling points right out the gate than water alone!
The "diethylene" in brake fluid absorbs far, far less moisture than the antifreeze one!
Therefore, it is used in our brake systems and won't freeze either!

Next time you plan to go "testosterone trekking" in a "low hanging" vehicle think about trussing up the brake fluid HDP bottle!
Try cutting the top cut off a quart or gallon milk or water jug can work!
They are made of Low Density Polypropylene.

You can stuff cleaning rags around it inside too, for that softer, dryer ride! (:)

Phil