1970 Datsun 1600 Master cylinder

Transmission problem
1970 Datsun 1600 4 cyl Two Wheel Drive Manual

My 1970 datsun pick-up clutch pedal is loose, I step on it and it goes all the way down, I cannot shift into any gear.
the truck was stored for about 5 years, when the clutch used to be fine.
i opened the reservoir and it was muddy looking.
what could be the problem?
when I took the cap off the reservoir and stepped on the clutch it squirts the fluid.
I kept doing it and added more fluid, until in got clearer.
but it is still the same.
is that normal?
what should I do?