I heard of this happening before and it was the slave cylinder, basically the inner seal had blown and the dust seal was retaining the fluid but because the fluid had moved to the dust seal the pedal was less efficient if that makes sense
it does to me
no doubt you have your checked your fluid level
the problem is its expensive because if you take the box off to repair it, you might aswell put a new clutch in
get underneath and pull the dust seal on the slave cylinder off, if fluid comes out, then thats your prob