I finally got around to fitting the new slave piston today and had an unforseen problem (and a couple of thoughts about why some folks may be having trouble with their clutches...) Anyway, the old piston popped out easy enough, so I applied a little oil to the new piston and quad ring, slid it in - and it stuck fast! I had to bleed the system to force it out. A closer inspection (and some careful measurment) showed that the bore was oval, caused by the mounting flange being very slightly concave. It looked like the sealing o-ring between the slave and motor wasn't seated properly at some point as it appeared to have been nipped, but luckily we have spare stuff like that on the farm! The stock piston, although not visibly worn is a touch smaller, so didn't stick in the bore. The damage to the o-ring was slight and wasn't apparent 'til I removed it, but the slave is pretty fragile, so it may have been enough to bend it. There was no alternative but to remove the cylinder, support the flange with a big socket and gently press the thing flat until the piston (minus quad ring) fitted properly. Luckily it straightened pretty easily. Assembly second time around went fine. I reverse bled the system, using a syringe on the bleed nipple to force oil (5W fork oil, 'cos I had some spare) up to the master cylinder and chase the air out. I pushed the piston all the way into the cylinder a few times get rid of the little bit of air that was trapped in the cylinder (you'll hear it coming out of the master cylinder relief port), then all I had to do was fit the cylinder and other hardware and set the oil level. I'll give it a good workout tomorrow. Thoughts / conclusions: 1) I should have inspected the cylinder condition more closely before attempting assembly. 2) The piston assy should move freely enough to allow the spring behind it to return it to the clutch pushrod. I believe this could be one of the reasons for suddenly "losing the lever". Perhaps the piston can be forced back by crankcase pressure or whatever. If the piston's not touching the pushrod you'll have no clutch. I also think this is why thin oil in the clutch system is important - the hose bore and relief port are small, so the lower the oil visco, the easier everything will settle. Perhaps a slightly stronger spring would help. 3)When refitting the slave check the o-ring for condition and correct seating and tighten the screws evenly so it pulls the slave onto the engine squarely. Oh, and don't trap the wire from the gear position sensor. Sorry for the long, bloggy post but maybe it'll save someone from a similar hassle. Edit: I hasten to add that none of the aggro was due to the new piston! It was dark by the time I finished, but I'll bed it in tomorrow and see if it's cured the occasional snatchy clutch disengagement I've been experiencing.