[SV650rules]

Sometimes the rubber diaphragm stays inside the cap when you remove it, on other reservoirs it stays in the neck of the reservoir and you have to pull it out to add fluid, there will often be a plastic filter in the bottom of the reservoir to stop any bits that may be in brake fluid ( who re-uses brake fluid ? ) or maybe a bit of rubber breaks off the cap or diaphragm and filter will catch it. John T is correct that fluid does darken in the system, this may not be from water, just aging / contamination of the fluid... You can clearly tell old fluid by it being darker, and sometimes by the smell.