A hydraulic clutch relies on pressurized fluid to amplify/transmit force, so whenever an air bubble gets into a closed hydraulic system, it can dramatically decrease the system’s ability to pressurize the fluid. That, in turn, can prevent the clutch from being able to fully disengage when you’re shifting gears.
Note: Hydraulic clutch systems are very similar to hydraulic brake systems, so the same hydraulic principles and service tactics apply to both. Keep reading if you want to understand how the presence of air (or a leak) can negatively impact a hydraulic system’s performance.
A not-fully-disengaged clutch can result in hard shifting, grinding gears, and increased wear on both your transmission and clutch.
The clutch connoisseurs over at Perfection Clutch made this brilliant video which explains the issue far better than we can.
They built a hydraulic clutch system into a visible “science fair”-style setup, which can be adjusted to introduce an air bubble into the system. Then, they attached a dial indicator to the clutch plate to show you exactly how the clutch movement is compromised by the presence of an air bubble.
The Perfection Clutch folks went to all this trouble simply to demonstrate the importance of a healthy hydraulic clutch system. So, be sure to periodically check your hydraulic lines, and master and slave cylinders for leaks and damage.
If you’re noticing yourself grinding gears or missing shifts more often, there’s a darn good chance your clutch isn’t engaging properly. If your clutch (or brake) pedal feels “spongy” or soft, then that’s another indication that unwanted air is present in the hydraulic lines.
You should also be replacing your clutch fluid periodically (it’s the same stuff as brake fluid). That’s because brake fluid is hygroscopic, which means it will naturally absorb moisture over time, making it less effective at resisting compression. (The results of having water in your hydraulic system are similar to having air bubbles in there.)
Check this out: