Bought my 90 about 8 weeks ago, the next day the clutch pedal went rock hard and would not go down. It then fixed itself somehow a day later?
A week later it did it again only this time my foot was on the pedal and it lifted itself and went solid.
So I thought it was a hydraulic issue, replaced master and slave but still solid.
So then replaced complete clutch with arm, worked great until last night, put foot on clutch to drive out garden and my foot raised itself and now it's soils again??

Have you made sure that the pedal has been adjusted for the correct amount of freeplay on the threaded adjuster otherwise fluid can become trapped in the system under pressure if the master cylinder piston cant retract fully. If that is ok then next time it goes solid get someone to keep pressure on the pedal and then open the slave cylinder bleed nipple, if the pedal then drops something is stopping the slave piston moving, if the pedal doesn't move then you may have something physically jamming the pedal?

The clutch pedal at rest should be about 1cm lower than the brake pedal.