OK, here's the skinny on brake fluids of the DOT 3 and 4 range. (as DOT 5 Brake Fluid is silicone based and another animal all together)
DOT 3 and DOT 4 brake fluids are glycol bases fluids and are by nature really good desiccants. Which means that they will absorb moisture right out of the air.
Over time, and especially in humid areas they will absorb large amounts of moisture fairly quickly.
This moisture in the fluid does two bad things.
1) A fluids DOT rating is based on it's boiling point. If you read the can, most times it will state the boiling point on it someplace. On even better brands of fluids it will state both the "dry" and "wet" boiling points. As when the percentage of moisture goes up in brake fluid it's boiling point goes down. Hit too low a boiling point for the conditions of your system and it will do just that, boil. Once the fluid has boiled it will have induced "air" into the system from the boiling, this air can and will do anything from make your brakes spongy to having no pedal at all!
2) Once high enough, it is this moisture in the system will begin to attack the metal parts in the system and cause all kinds of corrosion problems from the inside out.
This is why most auto makers, and now even bike companies that use glycol based fluids are recommending changes between 2 to 3 years.
Also, to make matters worse the "nice" little "V" BRP choose to have cut into the rubber brake fluid cap on our Spyders as one of their first "recalls" so the cap seal would not draw down into the reservoir and falsely push down on the low fluid sensor float. Well that "V" lets this system that should normally be "closed" have a direct "vent" to outside moisture laden air. This "vent" speeds up the process of moisture getting into the system.
You see rubber bladder type cap seals are normally supposed to draw down into the reservoir to take the place of the fluid level as it drops due to the pistons moving further out in their bores as the brake pads wear. This allows the system to not have a vacuum form inside of it from this fluid level drop, and yet it keeps the system sealed from direct contact without moist humid outside air.
Unfortunately, when this cap seal was the way it should be and was drawn down into the reservoir it almost immediately hit the float in the reservoir falsely causing it to trigger a low fluid limp mode. So the "V" was cut into this cap seal, IMHO to band-aid fix a true design short coming in the reservoir.
So changing the brake fluid may seem like overkill to some folks, but with this now "open" system it is cheap insurance from future brake system problems.
MM
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