The problem is caused by a plugged or covered compensating port in the clutch master cylinder. When the fluid expands from engine heat, the fluid pushes on the slave cylinder — causing the clutch to slip.
Below is a simplified illustration of a master cylinder:
The red arrow on the left points to the vent port a.k.a. the compensating port. This port allows fluid to expand back to the reservoir when the fluid heats up and increases in volume. This port has a tiny orifice and can easily get plugged from debris that has entered the system.
The right red arrow points to the inlet port a.k.a replenishing port. This port refills the system as the fluid cools and decreases in volume. This port is larger and presents less of a plugging potential.
The master cylinder piston is positioned such that when the piston is at rest the vent port is just ahead and the inlet port is just behind the lip forming the front face of the piston.
When the lever is pulled the piston moves forward with the lip of the seal covering the vent port. Before this port is covered, piston motion forces excess fluid back into the reservoir. If you look into a reservoir just as the piston begins to move you will see slight fluid squirt.
Once the vent port is covered, fluid trapped in the hydraulic system cannot escape provided there are no leaks in the system. Beyond this position the piston moves fluid to apply a force to the slave cylinder.
The master cylinder piston is adjusted at the factory so that, in it’s fully retracted state/starting position, the vent 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 free-play is necessary to prevent covering the vent port as the piston seal expands normally over time, and to avoid placing the piston seal rest location (where corrosion occurs) right at the vent port orifice.
Once adjusted, the manufacturer places a potting compound over the screw adjuster to prevent accidental or incorrect adjustment. I do not recommend adjusting this yourself.
If the potting compound is removed and the piston is incorrectly adjusted such that the vent 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 a common misconception that you can increase the piston stroke volume by adjusting the piston position and 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.
Another common reason for the vent port being covered is the installation of improperly sized aftermarket levers that interfere with the complete retraction of the piston.
Below is a simplified illustration of a master cylinder:
The red arrow on the left points to the vent port a.k.a. the compensating port. This port allows fluid to expand back to the reservoir when the fluid heats up and increases in volume. This port has a tiny orifice and can easily get plugged from debris that has entered the system.
The right red arrow points to the inlet port a.k.a replenishing port. This port refills the system as the fluid cools and decreases in volume. This port is larger and presents less of a plugging potential.
The master cylinder piston is positioned such that when the piston is at rest the vent port is just ahead and the inlet port is just behind the lip forming the front face of the piston.
When the lever is pulled the piston moves forward with the lip of the seal covering the vent port. Before this port is covered, piston motion forces excess fluid back into the reservoir. If you look into a reservoir just as the piston begins to move you will see slight fluid squirt.
Once the vent port is covered, fluid trapped in the hydraulic system cannot escape provided there are no leaks in the system. Beyond this position the piston moves fluid to apply a force to the slave cylinder.
The master cylinder piston is adjusted at the factory so that, in it’s fully retracted state/starting position, the vent 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 free-play is necessary to prevent covering the vent port as the piston seal expands normally over time, and to avoid placing the piston seal rest location (where corrosion occurs) right at the vent port orifice.
Once adjusted, the manufacturer places a potting compound over the screw adjuster to prevent accidental or incorrect adjustment. I do not recommend adjusting this yourself.
If the potting compound is removed and the piston is incorrectly adjusted such that the vent 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 a common misconception that you can increase the piston stroke volume by adjusting the piston position and 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.
Another common reason for the vent port being covered is the installation of improperly sized aftermarket levers that interfere with the complete retraction of the piston.
