There shouldn't be any particulates in the reservoir. The brake fluid should be replaced and the system flushed completely every couple years, especially if you live in a place that has high humidity. The system is not air tight, and as brake fluid is hygroscopic, it will suck moisture out of the air. This moisture will eventually contaminate the fluid to the point where it no longer functions properly. Its very simple to check the fluid using a small testing device available at many auto parts stores. Once you get up to about 2% moisture content its time to flush and refill.