At the risk of start another silicone/DOT3&4 debate, Silicone DOT 5 won't entirely prevent moisture from getting in. Moisture (water) is not soluble in silicone so any that gets in doesn't dissolve in the fluid but will usually sit around as little drops on the surfaces. If that little drop gets in the right hot place then the boiling point becomes 212 deg F. So you have to have a system that is completely and totally sealed. The advantage of the glycol based fluids is that any moisture that gets in dissolves in the fluid and lowers it's boiling point but it will never down get to 212 deg F since there will never be any pure water present.
