It sounds like a vacuum leak to me. I have been rebuilding a 72 and was told by a club member to check the clutch servo by pulling the vaacum line off, pushing in the rod by hand and then putting my finger over the tube the line attaches to. This makes the rod stay in if the diaphram is good. It worked for me, and mine works now that I have it running. If it is bad then CIP1 has a kit to fix it for $29.95. If it is good, look for leaks in the lines or in the cannister under the fender.
Emmett