Here is an important advisory note to remind everyone why it is important to periodically flush brake and clutch hydraulic systems every two years (or thereabout). See if you can get this picture permanently ingrained into your head.

When you do finally decide it is time to flush the fluid, consider these next pictures.

One of the problems with flushing fluids is to get rid of the old contminants that might still hang around after flushing. Even if there wasn't any muck in the reservoir, pouring a few ounces of fresh fluid into a few ounces of dirty fluid does not make fresh fluid. So it is a good idea to remove as much old fluid as possible first. Then get a bright small spot light (like a little pocket flash light) to look into the reservoir to see if there may be any of the muck present. If so, then seriously consider removing the top plate from the reservoir to get in there to clean it out. A paper gasket is pretty cheap, and you likely only need to do it once. Assuming you will form good habits of flushing the system periodically in the future, it should (in theory) never get that mucky again.

This goes for the rest of the hydraulic system as well. If there was muck in the reservoir, then there is likely muck in the slave cylinders too, in which case you should consider disassembling the slaves for cleaning (and maybe repacking rubber bits at the same time). And if you ever chagnge fluid type from glycol to silicone (or vice versa), then you should disassemble all cylinders for cleaning and repacking, as well as changing all hydraulic hoses. In general, rubber will absorb some of the fluid it is exposed to, and odd (sometimes unpleasant) things happen when exposed to another fluid type.