No matter how or where the water got in, it is heavier than the mineral oil brake fluid, so it settled into the lowest part of the brake system, your caliper. When the brakes got hot, the water boiled. Steam is a compressible gas. That's not what you want for brakes. That is why you lost braking only to have it return, after the brake fluid cooled.
Another possibility, is that your caliper O-ring is failing. Have you made landings on a wet runway or wet grass? If so, the caliper O-ring may have allowed the water to get in. Our aircraft calipers do not have an outer dust seal, as our cars and trucks have. Part of the dust seal's job is to protect the inner seal [only seal on your RV caliper] from debris and water.
I would suggest that you remove the caliper. Pop the piston out [using compressed air] and inspect the caliper bore. It may be corroded from the water [if it sat in the caliper for any length of time] Hone or polish out any corrosion you find in the caliper bore. If the piston has rust pits on it's circumference, it must be replaced. Pay attention to how the piston comes out. There is a right way [O-ring groove inboard] and a wrong way [O-ring groove outboard] to install the caliper piston in it's bore. I can supply photos of how it goes in, if needed.
Replace your caliper O-rings [size 2-218] with O-rings made of Viton. Viton has a much higher resistance to heat, than the stock Nitrile rubber O-rings.
Automotive brake fluids [except silicone DOT5] are designed to absorb any moisture which might get in the system. Wet automotive brake fluid's performance will be degraded, but will not become a "steam" engine, as your caliper did. This feature of automotive brake fluids helps protect the brake hydraulic system parts from corrosion, providing that the fluid is changed on time [every 2 years] to prevent the water build up from becoming excessive.
Charlie
