Clutch issue

Hello, I have a 79 vette with a 4-speed tranny and it was running fine. I went on vacation for a month and now that I'm back it seems the clutch pedal is really hard to press. Its not a hydraulic clutch as far as I can tell so I can't seem to figure out why all of a sudden it just sits for a month and then gets really hard to press. Its still seems to drive fine but because it just happened all of a sudden and it still works I don't think its an issue with wear.

Any ideas?

Thanks