I think you found the brake fluid reservoir.

If you have two caps on it its for the brake fluid.

Flushing it?

Hmm.

You'll need someone to help you do it.

You pump the brake pedal a few times and then open the bleeder valve on each corner one at a time while someone keeps pressure on the pedal.

When it stops coming out you close the valve and release the brake pedal.

Top up your brake reservoir and repeat for each corner until you only get clear fluid coming through.

You'll need 2 bottles probably.

Make sure the reservoir doesn't go below half full.

 

All brake/clutch fluid turns dark after a while. It's the rubber parts going into solution. Basically suspended rubber dust. Most of those fluids are hydroscopic so the darkening due to age is a good indicator that it's time to change the fluid.