Glenn Goodspeed (December, 1996)
Some folks claim that because silicone brake fluid (Dept. of Transportation #5) does not absorb water, it will keep moisture from collecting in the brake hydraulic system. DOT 3 and DOT 4 fluid, being alcohol-based, may indeed absorb water from the air, but not fast enough to warrant switching to DOT 5.
Even a hydraulic system that contains silicone fluid may collect water when moisture condenses on the roof of the master cylinder and drips into the fluid below. Since water is heavier than silicone fluid, it sinks to the bottom of the cylinder and is sucked into the rest of the system. Over time, the whole system may become polluted in this manner.
If you doubt that this can happen, I invite you to open a transmission that has sat unused for several years. You will be able to pour out a quantity of condensed water in addition to the transmission oil.
I believe the only way to make sure your brake lines and internal parts are clean and water-free is to bleed the system with fresh fluid about once every year. It only takes about an hour with a one-man brake bleeder and about $5 worth of fluid.
If you just can't resist trying silicone fluid, remember that you must flush the hydraulic system completely with the new fluid to remove all the old fluid. It is a good idea to replace all the seals and clean internal parts in the calipers and cylinders, too. A lot of work for a dubious return.