1986 BMW 325 6 cyl Two Wheel Drive Manual 218000 miles

I'm sure the pressure plate & disc is fine but over time the pedal doesn't release the clutch enough to shift easily. I saw no leakage at the slave cylinder. The pedal had more and more free play. Under the dash I noted the pedal return spring assy. was falling apart (due to the free play?). I removed the master cyl. and am wondering how I can test it. Where most master cylinders have a place to fill with brake fluid, this one just has a rubber boot/plug with a hole in the top. What do I do next??? When I actuate the rod, nothing comes out of the end that connects to the slave cylinder.
Any help would be greatly appreciated.

How do you know the clutch is ok? I would replace the master cylinder and see what happens.