The only problem with DOT 5 fluid is air. It's nearly impossible to get all the air bubbles out of DOT 5 fluid. so be careful when pouring it into a new system, to keep introduced air bubbles to an absolute minimum. Jostling a sealed bottle of DOT 5 fluid introduces air bubbles into the fluid during shipping. A big bubble will seem to go away, but it's actually breaking down into smaller and smaller bubbles. But I promise you, those tiny bubbles will find their way back to one another after a bit of time. And while DOT 5 won't absorb water like DOT 3 fluid, this is actually not a benefit. If moisture is introduced into a DOT 5 system (ever check your brake fluid on a rainy day?), the moisture will drop to the lowest point in the system and pool there. If that happens to be a caliper, then the water will boil at a much lower temperature than the fluid. This can lead to complete brake failure, because steam will pressurize the system. Suddenly, your high-boiling point DOT 5 system is air-locked at a much lower temperature than a DOT 3 system. Additionally, a DOT 5 system that gives a rock-hard pedal at sea level will give you a spongy pedal at high altitude.
DOT 5 is really nice, because it isn't a paint-killer. But when it comes right down to it, how many of you have ever destroyed paint by dumping brake fluid on it? I've never had it happen, myself. I was always taught to pour the fluid into the master cylinder reservoir, not on painted surfaces.
DOT 5 is also really nice, because... Well, there has to be another reason, doesn't there?
Oh, yeah, DOT 5 is really nice, because you get to run fresh fluid in your car all the time, because you will need to flush the brake system on a regular basis to get rid of pooled moisture in the system. You are flushing your DOT 5 systems each year, aren't you? DOT 5 fluids seemed to be a natural for race cars (although most racers avoid the stuff like the plague), but remember that most race cars are constantly undergoing maintenance your street car will never see. Just because something works well on a race car doesn't necessarily mean you want to use it on your street-driver.
If you're concerned about moisture lowering the boiling point of your brake fluid, start out with Castrol LMA (Low Moisture Absorption). But be sure you are buying it from a suppler that rolls their inventory over very rapidly, because LMA comes in a plastic bottle. And yes, Aunt Matilda, water will manage to get through that plastic bottle. If you're just looking for a reliable brake fluid, buy a metal can of DOT 3 fluid (at a mere fraction of DOT 5 fluid's cost) and be careful you don't pour it on your nice paint. While your buddies are flushing their DOT 5 systems each year, you can be out, enjoying another day of cruising. You'll also have the added benefit of being able to get brake fluid anywhere, should you need to do some roadside brake repairs.
