I would like to pass on a tip which may spare someone the grief I had recently due to a hydraulic leak.
Some months ago, shifting my BMW became increasingly difficult to the point where I had to turn off the engine when stopped in order to get into first gear. Trapped air in the clutch slave cylinder seemed to be the problem, but bleeding only corrected the situation for a short time. A new slave cylinder was installed and the clutch worked fine, except that the red brake-failure light came on indicating a low level of hydraulic fluid. I was puzzled by the fact that although I had added fluid more than once, there was no evidence of leakage on the garage floor. Suddenly the answer came in the form of hydraulic fluid all over the floor carpeting near the firewall. Fluid was leaking out of the clutch master cylinder and collecting in the pocket where it is. attached to the firewall. When this pocket filled, there was no place to go but inside the car!
To clean up the mess from the rug I used isopropyl alcohol and soapy water. It took a lot of both, plus time and hard work. The moral is don't keep adding brake fluid to your car without knowing where it is going!
Author: Nat Murray
Generally the clutch slave cylinder needs replacement when it is leaking Pull the protective boot back. If fluid oozes or drips, the cylinder is passing air into it Likewise, there is a protective boot on the clutch master cylinder and the same applies. Slave cylinders which have been exposed to severe winters or great changes in temperatures usually will fail at about 70,000 miles. The reason that the unit is replaced and not rebuilt is that it is nearly impossible to "rebuild." When checking the unit with a bore gauge you will discover that the bore varies a great deal and is tapered. The result is that the seals just installed will wear out in several months. The practice of honing the cylinder is also of little avail since the seals provided are the same size as the ones originally installed.
Author: Tech Ed