I agree with Gordon. Moisture laden air is bound to get into the system over the course of a couple of years. Hydraulic fluid is very hygroscopic and will absorb the water from the air (that's why you always have to use fresh fluid rather than stuff from a can that's half empty).
The 'stuff' could just be emulsified fluid, but it's a good idea to flush it out because when the fluid gets hot the water in it will vapourise, and because you can't compress liquid but you can compress gas, the lever will get very soggy.
Not so much of a problem with your clutch, but with your brakes