The master cylinder piston is adjusted at the factory so that, in its fully retracted state/starting position, the fluid return port
to the reservoir is completely uncovered.
Essentially, until the piston moves far enough to cover the port, fluid can flow from the cylinder to the reservoir and thus the fluid is not pressurized sufficiently to cause the lever to generate the forces required for clutch release.
Properly adjusted, there should be a 5-10% dead-band of the full lever movement before there is pull resistance felt at the lever. This freeplay is necessary to prevent covering the port as the piston seal expands normally over time, and to avoid placing the piston seal rest location (where corrosion occurs) right at the return port orifice.
Once adjusted, the manufacturer places a potting compound over the screw adjuster
to prevent accidental or incorrect adjustment.
If the potting compound is removed and the piston is incorrectly adjusted such that the return port is completely or partially blocked when the lever is released, the system will remain partially pressurized for a time, and the clutch engagement will be incomplete, causing slippage and premature wear
. Similarly, the same mis-adjustment of the brake master cylinder leads to brake lock-up when the brake fluid heats up.
It’s been sugested here that you can do this adjustment yourself by checking for a squirt of fluid into the reservoir when you initially pull the lever, i.e. if you can see a squirt, it’s OK. However, keep in mind that you’ll still see a squirt with a partially blocked port. Clutch release problems are best solved by the proper selection of slave and master cylinder sizes. Reducing the amount of freeplay using this approach will allow you to move a little more fluid with the same amount of lever movement, but I don’t recommend it.