Water in brake fluid doesn't turn to steam and then air (turning water to steam requires heat and/or very low pressure, turning steam into air not possible, air being mainly nitrogen and oxygen).
And what do you think results as a by=product of braking friction...? Heat... lots of it. I stand by my statement "red"
Brake fluid is hygroscopic (absorbs water). The issue is that the water corrodes the internal surfaces of pipes and cylinders.
That is NOT the case with copper/nickel brake pipes actually...
Air in the system is introduced when replacing hydralulic parts, when the reservoir level gets too low or sometimes through connections on pipes/hoses not being tight enough. The car has had new discs and pads but these of course are not hydraulic parts.
Any sponginess without running the engine is usually air and needs bleeding. Some systems can be hard to bleed effectively.
With pedal depressed then starting the engine, the pedal should go down a bit as the vacuum kicks in.
If the pedal travel is getting near the floor in normal use, that is dangerous and needs sorting urgently. No kidding?
red.