Emergency braking doesn’t have much to do with moisture level. Fluid cannot reach such heat levels from one hard stop.
But several braking attempts going downhill in the mountains could influence brake performance especially if emergency attempt is made after braking harder several times.
Wet boiling point of brake fluid is measured at 3.4%. That is not tall order and average DOT3 has wet boiling point below 300f. Prestone Synthetic DOT3 has wet boiling point 284f. 5-10yrs old brake fluid is well below that.
You’re right, mountain driving can test even a properly maintained system if one is not careful.
But emergency braking and I’m talking from highway speeds of 70-80mph (should’ve said that in my original post) for sure can expose a weak and under-maintained system as well.
Most situations involve already warmed up brakes from normal driving or stop and go traffic. It doesn’t take much to increase the temperature from there especially at the calipers.
I’m not talking about the pedal going to the floor suddenly, but a gradual decline in braking performance. Something that is not really noticeable in everyday driving.
And an increase of several feet of braking would not really be noticeable, but can definitely mean either stopping on time or hitting whatever is in front of you. But if there is no outright failure, nobody will even question the brakes.