[Excaliber]

Slave cylinders don't use an O ring, they use a 'cup'. Their failure is generally a result of the edges of the cup literally wearing out over time. Premature failure can occur if the surfaces of the cylinder that the cup moves back and forth in are rough (not smooth). This can happen if moisture gets in the bore and 'sits' there rusting the bottom of the bore and 'tearing' the rubber cup next time you push the clutch in! Result is sudden failure of the slave cylinder due to leaking fluid. Drive home without the clutch (as Jamo noted) and buy another one (their cheap) two bolts, a bleed and your done! Beats the heck out of dropping the trans!