The clutch pedal of the car (Fiat Punto, 1246cc) has lost its tension, it went all the way down when I pressed it with no resistance at all. I can't change the gear when the engiine is on. I spoke to a mechanic who told me the car doesn't use a clutch cable but mentioned something about the piston needing replacement. My question is what do you think is wrong and what does the piston do? thanks