Theory on why clutch bleed can be difficult - PeachParts Mercedes-Benz Forum

gastropodus · #1 05-14-2012, 04:29 PM

I changed out all the hydraulic components on my 240D clutch this weekend, and spent a fruitless afternoon trying to bleed the system. I'm not the first to have this problem, judging from the number of frustrated postings!

I'm familiar with the oil can video, and I was attempting to do a bottom-up bleed with something similar: what I did was drill holes in the lid of a peanut butter jar, mount some barbed fittings in the lid using epoxy, and then hook this up to an air pressure source (a dry garden sprayer, if you're curious). The idea is similar to the oil can: the jar was filled half full of brake fluid, and there was a pickup hose going through the lid such that as air pressure was introduced to the jar it would push fluid out a hose. This hose was connected to the clutch bleed nipple. So, pumping the air pump pushed air into jar, air in jar pushed fluid into clutch system. Or so goes the theory.

In practice, I just couldn't get fluid to go into the system at all! It leaked from the slave cylinder nipple, even after I introduced a hose clamp onto the hose to hold it more securely onto the nipple. It was as if there was something blocking the circuit that should be free going all the up to the reservoir when the clutch pedal is not pressed. I checked the eccentric shaft at the top of the clutch master cylinder; it was set to the position that should push the rod into the cylinder the least.

Finally today it hit me: I recalled that when I bolted the master cylinder into place, I had to force the rod down a little bit to get the the bolt at the end of the rod to line up. I'm guessing that the little bit of movement on the rod closes off the inlet port on the master cylinder so that now the master cylinder barrel and the slave cylinder barrel are a closed system - no place for air or fluid to go!

So, the master cylinder came with two rod lengths. I chose the rod length that I thought most closely matched the one I took out, but I wonder if that was the longer of the two rods... I probably need to try the other, shorter rod length.

Kurt