Try to bleed the slave a bit. The slave in my '80 would get moisture in the fluid and get a little short on stroke. It still worked just not ehough and 1st gear would be hard. You can gravity bleed them just crack the bleeder and top off from the res until fresh clear fluid comes out the slave. I ended up switching to synthetic brake fluid and it hasn't given me a problem since.
Might not be your problem but it's worth a try before you suspect something more serious.
