This may seem irrelevant but I use DOT 4 in my race bikes (motorcycles) and it works fine. DOT 5 (sliicone) is great so long as you can afford it and want to start fresh. The only reason for it's existence is because of the need for higher boiling points to meet the demands of modern braking systems. It was never intended to be used in the resto business even though it has worked out well in that regard. Half a dozen to one and six to the other. DOT 4 will never be a problem with taking on water if you live in the west (dry climate). If you live in the rain forest up here in NY the constant cycling of temperature causes excessive condensation and evaporation. This is where the water comes from in the brake fluid. Almost can't be avoided as the system is vented purposely. Throw in some heavy braking and more temperature cycling of the brake system and the problem is exagerated even further. I think in most of our old cars this is rarely a problem as they are stored nicely and not usually driven hard.
DOT 5 Silicone fluid does not enhance in any way the braking effectiveness of your system. It is a fluid and simply transfers motion. You could actually use water, oil, or even grapefruit juice to do the same job but it goes without saying the ancillary problems would be significant.
Anyone who has ever handled brake fluid noticed the unique texture and feel of this fluid. It's the glycol (basically sugar) that gives it this property. At the same time it's also why it is soluble in water. Not much different from conventional anti freeze. This is where it gets it's raised vapor point from. A double edged sword.
Silicone is synthetic (man made) from chemicals that are not organic. This is where it gets it's rasied vapor point as well. Keep in mind also that the violet color is a dye added to the fluid to identify it to the observer so that you do not add conventional fluid to a silicone system.
Just my ramblings for the evening.
