[Shop-talk] Weird clutch petal

[Robert Nogueirao]

I have a  hydraulic clutch, Girling 5/8” master and a internal slave that fits over the input shaft. Both are new.  Problem is I have a really good feeling petal, no leaks and good operation 90% of the time but occasionally when you depress the clutch to change gears the petal feels soft and will not fully release the clutch. Letting the petal out and again depressing it and the feel and release are again normal. Reservoir shows no lost of fluid.  
Since the slave is mounted on the input shaft and operates directly on the throw out bearing there is no linkage to cause the problem.
Any suggestions as to what could be causing this problem?
Bob Nogueira