As another pointed out, the reservoir is vented, allowing ambient air to
come in physical contact with the fluid. If the fluid is hydroscopic, like
DOT3 &4 fluids are, the water making up the humidity in the ambient
air will be pulled out it, and into the fluid. Obviously, this is a non-issue
for non-hydroscopic fluids like silicone based DOT5.
Active water spray is a different matter. Driving in the rain can actively
spray water onto the calipers, and it can spray it onto the master cylinder
reservoir. So can washing the car with a garden hose. In this case, water
is getting into the system mechanically by being forced in, and it has nothing
to do with the hydroscopic properties of the brake fluid. All brake fluids
would
be affected equally. For automobiles, it's normally a very minor source, but
on
things like motorcycles it can be quite significant.
In any case, with proper brake maintenance, water in the system is not a
problem, regardless of the fluid used. If contamination is a problem, it's
proof
positive of poor maintenance.
Many of us run brakes in quite poor condition actually. So detecting
a change in pedal feel when switching to a silicone based fluid is difficult.
Especially since it is normally accompanied with a complete hydraulic overhaul.
It would be far better to do the overhaul and start with DOT3 fluid in
this new system to establish a baseline, then purge and switch to DOT 5 for
comparison. Otherwise you are comparing an old (poor) mechanical system
using DOT 3 fluid with a new mechanical system using DOT5 fluid. You
are not simply comparing fluids.
To draw a conclusion about brake fluid from the above scenareo
would be as foolish as the people who conclude DOT3 &4 fluids never
needs to be changed because they've gone X years without changing
theirs. They do get away with it, because of the superb engineering
in many automotive brakes. It does not make their conclusion correct.
To that end, your needs end up depending on how hard you drive, and
how well you want to stop when you drive hard. Some folks show up
and race on oem pads, others run on things like Porterfield. Fwiw,
I've seen many cars with oem pads and silicone fluid, but I don't think
I've ever seen a car with Porterfield pads using silicone fluid.
If silicone based DOT 5 fluid does all you need of it, then it's the fluid
for you. It doesn't mean it's the fluid for everyone.
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