ok. don't panic ... yet. :)  this happened to me in my 80 bus, and it
was just the cold weather affecting dirty old brake/clutch fluid.
get it inside somewhere and bleed ALL the old fluid out, replacing it
with fresh stuff. don't forget the all-important clutch slave cylinder!

what happened (according to the mechanic) was that the rubber seals in
the master/slave cylinder got really cold and didn't expand as they should
and some crud from the dirty fluid got lodged in there and held the seals
open ... effectively meaning NO compression from the seals. :(

anyway, after replacing the fluid, i had a "new"clutch! worked great. so
change that fluid every year!! :)

joel