Clutch Not Working
 

I put a new radiator in my 1996 Miata yesterday without a problem (I think) but on the test drive my clutch stopped working. The pedal went straight to the floor with no pressure. I had enough speed and coasted to a nearby parking lot. I checked the Master Cylinder reservoir and it is almost full, though it does look very dark.
I got the car towed home (this was at night) and left it till the morning to work on it.
I checked for leaks and didn't see any visible. I checked the boot of the slave cylinder and it did not push out any fluid. I had my son work the pedal while I watched the piston. The piston moves a little but not fully.
I opened the bleed valve and had my son press the pedal. The fluid squirted out with pressure.
I tightened the bleed valve and had my son continue to press the clutch. Still the piston only moved slightly and never enough to engage the clutch.
Does this prove the slave cylinder is the problem and has a slave cylinder failed without a leak in the hydraulics?
Thanks for any advice you can give.

It doesn't prove much. The master could be worn out and simply unable to produce any serious pressure (beyond squirting fluid).

Slave cylinders are cheap and easy to replace though if you want to start there. You can find them for under $20.

Well I went to the parts store (Auto Zone) and bought both the Master and Slave Cylinders (they were $20 each). I decided to put the Master Cylinder in first, mainly because there were no leaks at the Slave Cylinder.
Upon putting the new Master Cylinder in and filling and bleeding the line, the clutch is working normally now.
I will keep the Slave Cylinder for now and see if it acts up in the short term.