Clutch pedal stucks (Quattro Coupe '87)

[datalapio at kolumbus.fi]

Hello,

I have a problem with my recently bought '87 quattro coupe 85 (4k). The clutch pedal stucks in the lower position. The fault develops slowly: the clutch contacts lower and lower until the pedal stucks in the lower position and does not return up anymore. The gears cannot be changed so the clutch is not slipping or disconnected. 

I changed the work (master) cylinder but now the fault appeared again. Is it in the other (slave) cylinder that is faulty, or is there air in the system or is the clutch itself that causes this behavior? The fluid level is ok in the clutch system.

Tuomo