The slave cylinder on your Audi S4 is used to transmit hydraulic pressure from the rod of the slave cylinder to the clutch fork, to disengage the clutch from the transmission. When the slave cylinder fails, you will have a hard time changing gears because the clutch won't be releasing completely. When this happens, replace the slave cylinder so that you don't damage your transmission and clutch.
Jack the S4 and place it on jack stands.
Disconnect the clutch hydraulic line using an 8-mm wrench.
Remove the two bolts holding the slave cylinder onto the transmission with a ratchet.
Place the new slave cylinder onto the transmission, and tighten the bolts with a ratchet.
Connect the hydraulic line to the slave cylinder with a wrench.
Top off the clutch master cylinder with brake fluid.
Bleed the clutch hydraulic line (air was introduced into the system when you replaced the slave cylinder).
Attach a clear rubber tube to the clutch slave cylinder bleeder.
Push down on the clutch pedal and, while pushing down, count, "1, 2, 3." On 3 say, "holding." This will inform your helper when the clutch pedal is fully depressed.
Instruct the helper to use an 8-mm wrench to open the slave cylinder bleeder when the clutch pedal is being depressed. The helper will open the bleeder at count 1 and close shortly after the clutch pedal is fully depressed.
Repeat Steps 2 and 3 until the assistant doesn't see any bubbles of air going through the clear rubber hose. At that point you should feel the clutch pedal to be a little harder to press down than before.
Lower the S4 to the ground and test the clutch system for proper operation. If the clutch remains "mushy" or "soft," repeat the process until all the air has exited the system.
