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Just a few short days ago we had a delicious discussion about why reverse gear crunches when engaged, which lead to a discussion about how a clutch wears, pedal travel engagement/disengagement, slave and master cylinder relationship and so forth. Well, some things were said that weren't right to my thinking, so I want to go back and rehash. I'd like to know if it's just me not understanding.
First of all, let's determine that it's the slackening of the PP springs against the worn, thinner disc surface during clutch engagement that causes slippage (usually with a howling screechy judder). As the disc wears, during engagement (foot completely off clutch pedal) the finger springs are pushed that much more into the slave cylinder, with the throw out bearing serving as the go between, pushing against the piston. As far as clutch pedal travel goes when a clutch begins to wear, disengagement would be sooner along the pedal travel toward the floor because the pressure needed to disengage is that much less than with a new clutch disc. But we're talking a small amount of difference since pedal travel regardless of the disc wear is determined more by the design and functional interchange between the master and slave cylinders.
It's all about how much fluid has to be pushed into the slave cylinder interior chamber as the master cylinder moves. If the slave fluid chamber volume is equal to the master cylinder chamber volume,* then any movement/change inside the master would result in an equal amount of movement in the slave cyl. Because the slave piston position changes during engagement as the clutch disc wears - pushed in more, less fluid volume internally - the master cylinder piston has to move that much less to move the slave piston outward to disengage the clutch disc from the flywheel. We're still talking a very small amount since disc wear between a new and used clutch disc is maybe 3mm.
* My guess is that the fluid volume chambers of the master and slave cylinders are not equal so as to make it easier to press the PP finger springs, with the slave cylinder having a larger volume compared with the master. How much larger I don't know since I'm not a hydraulics specialist. I'll guess somewhere in the neighborhood of 2 (slave) to 1 (master). Master cylinder moves 2 inches, slave piston moves 1 inch. This still doesn't change the issue of whether it takes more or less pedal travel to disengage the clutch as the disc wears.
There are other reasons why it can require more pedal movement to disengage the clutch. One is if there is air anywhere in the clutch hydraulics. The other is master cylinder wear and condition. Suppose the internal surface of the master cylinder is corroded or pitted along where the seal passes. When the seal passes over those pits surface, fluid pressure lessens as some fluid flows back toward the reservoir side of the piston instead of moving toward the slave cylinder. A worn master cylinder seal only adds to the back flow.
Also, as the cylinder interior surface wears/pits causing more movement necessary to engage the clutch, the seal has to pass over a surface area that hasn't seen friction. That "virgin" surface is often where contaminants damage the interior surface by first attaching themselves to the surface, then eating away at the metal and morphing into pits. So the seal now has to travel across a "moonscape" surface of hardened "pre-pit" bumps, as well as fully formed pits. That's when sudden failure can happen in the form of the seal becoming shredded at that point. If anyone has refurbished an aged master cylinder and has seen the gunk (decomposing fluid, moisture, decaying rubber seal material, decaying metal, general shmengy), that accumulates in that virgin territory, you know well what I mean.
Prior to complete failure, a corruption of the master cylinder, including seal wear, will cause limited fluid pressure - it works up to x psi, but not past that point. All of that increasing pedal travel to disengage the clutch. If slave cylinder seals are worn or the piston surface on which the slave seal rides is pitted or corroded, there will be a tell-tale leak since the fluid has nowhere else to go at that point but out of the system and onto the ground.
This goes back to why reverse crunches. If either air is somewhere inside, or the master cylinder seal is compromised, then regardless of pedal travel, that can cause the clutch disc to not fully disengage. A binding pilot bearing only adds to the problem.
Make sense?
posted by 207.200.116...
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