TT250 odd front brake issue

[oddthought]

Originally Posted by '16 TT250

Maybe someone with an actual education on this stuff can provide more info.

My understanding is that a hygroscopic fluid is one that can attract and hold water molecules in suspension. Water molecules (humidity) enter the system by coming in through the rubber (permeability) or by breathing if it's not a sealed system. The brake fluid can't make water, it has to get in somehow. Therefore I theorize that neither fluid can increase the amount of water in the system, only how much water that fluid can hold in suspension. It would seem that a more hygroscopic fluid might actually be better. A non-hygroscopic fluid such as silicone won't hold water in suspension therefore any that gets in will remain as water in direct contact with the brake parts which could lead to corrosion. Since the water will sink just like water in oil it will accumulate at the lowest point in the system and will be hard or impossible to remove by just bleeding/flushing.

Just my theories, I'm not claiming any of this to be fact and haven't researched any of it yet.

I agree with you on most of this! I'm no expert on brake fluid specifically, but I am a chemist for a living, so here's my take after a bit more research: the evidence is pretty lacking that there's any significant difference between the two fluids in terms of how hygroscopic they are. I had heard somewhere that there was a difference, but looking into it, I can't find much if any data to back that up. Since I can't find the data, I'll assume they're equal in that regard moving forward. If that's the case, I might as well use the DOT4 for the higher boiling point, but if I only have DOT3 handy, I'll just use that.