Oh that’s just the slave cylinder from my e36 M3.

“Is it supposed to be in pieces like that?”

No, no it’s not.

“Well, why is it?”

Funny story, I was replacing the clutch on my M3 (among other things) and decided as a preventative measure to replace the slave cylinder.

In case you didn’t know, the slave cylinder is the hydraulic part that engages when you press on the clutch. The clutch pedal is attached to the clutch master cylinder. The master cylinder pressurizes brake fluid when you push clutch and the fluid goes through the line and pushes against the slave cylinder. The rod on the slave cylinder then engages a pivot fork which is attached to a throw-out bearing on the input shaft of the transmission. When you press on the clutch, this is actually releasing the pressure that holds the clutch disc against the flywheel.

Now that we have discussed what a slave cylinder does, I guess we can continue on with why it’s in pieces.

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Well, I was wrapping up the clutch job on the M3. Since I decided it was necessary to replace the slave cylinder, I needed to bleed the air out of the clutch lines. The process is very similar to bleeding brakes. You need to get the air out of the system for it to work properly.

I inserted the slave cylinder back in the transmission and hooked the lines up to it. I started going through the bleeding process (opening bleeder valve and pushing in the clutch pedal, closing, pressing the pedal, repeat). On the 2nd or 3rd round, after closing the bleeder valve, I heard a pop and the pedal went soft. I looked under the car and there was fluid draining out of the front half of the transmission case. After turning red in the face and swearing for a few minutes, I decided to pull out the slave cylinder to see what had happened. To my horror, only half of it came out. That means the other half had separated somehow and was sitting in the bottom of the transmission casing. The hole is about an inch wide, so retrieving it wasn’t going to be possible.

After getting red again and swearing for a few more minutes I decided to assess the situation to see how this happened. The slave cylinder operates pretty much like a brake caliper. For example, if you pressed the brake pedal when the caliper wasn’t attached to the car, the cylinder on the caliper would eventually pop out. The slave cylinder also needs something to prevent it from coming apart.

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So what happened? Well, the rod on the slave cylinder isn’t totally rigid. It pretty much is just secured with a rubber boot and can be moved up or down (in the photo above, I had already pushed the cylinder back into the housing). When I put it back in the car, it must not have been totally straight and the rod must have slipped past the pivot fork. When I began bleeding the system, there was nothing to keep the cylinder from popping out of the housing and so it separated, dumping all of the fluid out and filling the transmission up with slave cylinder bits.

So, I went into a clean-up frenzy and pulled the exhaust, driveshaft, shifter linkage, a few sensor wires, removed the starter bolts, and then removed the bolts holding the transmission to the car (the top few almost impossible to reach. I slipped the transmission off the car and found the missing parts. I wiped all of the brake fluid off of the inside of the transmission and then went on another frenzy to put all the parts back together.

I meticulously put the cylinder back into the housing and pressed the metal clip that holds the rod back together. I then put it back into the transmission, this time ensuring that there was adequate tension on the spring mechanism inside the slave cylinder. I bolted everything up, re-bled the system, tested the clutch to make sure it was working properly and performed the obligatory victory dance that every backyard mechanic does when they remedy their own fuck-up.

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TLDR: Be careful when you are working on your car.