1961 hydraulic clutch problems

[gmcbigbl]

Just got my new (to me) 1961 Apache. Its pretty much rust free except for lower fenders and hood which is the later one for 1962 to 1966 trucks.
It has a hydraulic clutch problem, that I can't quite figure out yet. Initially checked dual master cylinder and was MT. Added brake fluid
and did bleed the actuator (slave cylinder), and checked all around for any leaks and didn't find any.
If I try to put the tranny into gear while running, it just grinds away and won't go into gear. If I pump the clutch pedal several times (I guess to build
up pressure) it will go into gear and shift. After a minute or less clutch won't shift anymore and depressing it does nothing-truck keeps
going and stopping with the foot brake kills engine, because tranny is stuck in first and can't remove it. Pumping clutch pedal does help
for a shift or two. but loses pressure fast??? Clutch grabs flywheel very low on pedal(about 2" from floor) and seems to fade away quickly.
Sounds like a leak somewhere, but I can't find any and brake reservoir remains full?
Any suggestions would be a great help.