Wanted to take the time to correct some of the above information, with enough detail that those who may not be familiar with the clutch piston assembly can avoid the same (silly?) mistake I made! John's third bullet in post #35 hit it on the nose. I'm not sure what part of the push rod ('04 P/N 22884-MB0-006) indicated that to him, but it is indeed 'handed' or indexed, both by the rounded end being different than the flat end, but also as shown in the photos below, the rounded end is a larger diameter (0.206") than the flat end (0.192"). In addition, the pin length with the flat end is approximately 0.65", while the length with the round end is that long just to the start of the rounded portion, so is slightly longer to the tip. Both boot seal shoulders are the same width but because of the difference in the end lengths, the different orientations would place the boot seal a bit closer or further from the larger diameter shoulder inside the master cylinder or not let the large diameter end seat fully against the snap ring. I'm not sure what difference that would make but perhaps it affects the flexing of the boot over time?
plunger length difference (L round end / R flat end); doesn't appear to be significant
bushing seat difference (L round end / R flat end); definitely is significant!
plunger diameter (L round end / R flat end); slightly different
More importantly, the clutch lever bushing is not large enough to let it fully seat onto the rounded end; it only seats partially the distance it does on the square end, thus increasing the overall length of the bush/push rod/piston assembly if installed backwards. Interestingly enough, I felt this when pushing the entire assembly (rounded end into the bushing) in far enough to insert the clutch lever pivot bolt but it didn't register at the time. Doing so with the extra length due to the plunger not seating fully into the bushing pushes the primary cup (on the end of the spring) past the compensation port (the small hole) in the master cylinder floor. You will notice this by there not being a 'spurt' of fluid in the first bit of travel when pulling in the clutch lever. It was there before but it wasn't there after reassembly. Those two things should have given enough of a clue as to the incorrect orientation. The other indication was the increasing pressure in the clutch system due to the compensation port not being able to provide any relief to the point where all play was lost in the clutch lever and eventually the slave cylinder pushed the clutch rod in far enough that the plates just spun against each other vs providing any drive force.
External boot, properly installed toward rounded end to fit inside clutch master cylinder. There isn't a groove (that I could detect) and instead the large diameter ring is just solid enough to be an interference fit against the large outer master cylinder bore.
The snap ring looks like it's not fully seated, but every time I put it in, it looked the same. As you can see from the picture of just the ring itself, the width is not the same across the entire ring but is wider opposite the opening. Taking that into account for the difference compared to outer edge of the the piston core in the end-on photo, and the fact that pushing it in further against the washer/piston didn't make it seat any deeper, I assume that it's fully seated and looks like the photos below. Larry also pointed out that they also have an orientation. The flat side should be seated opposite any pressure to eliminate potential for the rounded side to collapse the ring under pressure and let it unseat reference the mating groove. With the flat side, there is no shoulder to work against and the entire surface mates with the flat machined groove.
All that to say, that had I just paid attention to the last two photos in post #18 that shows prior to disassembly to fix the leak, the squared end should go into the clutch lever brass bushing and the rounded end goes into the master cylinder/piston or test fit the brass bushing onto each end of the plunger to see if they both fit the same, or...well, you get the point.
Thanks to John and Larry (and everyone else) for not calling me out as the obvious 'poor' mechanic and instead just helping me/us work through the problem systematically and logically. If only I'd applied that in the first place!
Ride safe everyone!
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(small metal cap removed for clarity)
