With all the emphasis placed on checking and changing your brake fluid, it’s easy to overlook the fact that your hydraulic fluid needs to be serviced regularly too.

That’s because a hydraulic clutch and a brake system rely on the same basic principles of hydraulics and share many similar system components. In fact, many hydraulic clutch systems use standard spec brake fluid—but check with your vehicle’s owner’s manual to make sure.

That means your clutch fluid is hygroscopic too, which is a fancy way of saying it sucks up moisture from the air over time. And, since water compresses more than hydraulic fluid, water-saturated fluid can result in a spongy pedal feel. (And that’s true of both your brake and clutch system.)

So…it’s important to check your hydraulic clutch fluid level regularly, just as you would your brake fluid. If you’re not sure how to do that, read on…

Here’s the slave cylinder for a vehicle equipped with a hydraulic clutch, snugged in tight between the firewall and the inner front fender. The hard line at the front exits to the clutch itself, while there’s a larger, partially-hidden fluid line that extends up to a divorced fluid reservoir near the top of the engine bay. (Image/OnAllCylinders – Paul Sakalas)

Understanding Different Hydraulic Clutch Types

But before we get too far, let’s dial back and talk about the types of hydraulic clutch mechanisms out there.

There are essentially two types of hydraulic clutch actuator designs that you’ll likely come across. First, there’s the pushrod style, which is basically just a piston that moves a linkage attached to a clutch fork. Pushing the clutch pedal actuates the pushrod, moving the cylinder and engaging the clutch fork (kind of like a traditional all-mechanical clutch).

The other actuator design is becoming increasingly common in modern vehicles. The clutch is actually housed in a bearing that fits around the transmission’s input shaft. When the clutch pedal is pressed, it actuates a hydraulic throwout bearing that pushes against the clutch diaphragm to disengage your clutch.

Regardless of the design of your clutch mechanism, they both rely on the same basic hydraulic principles, so the information below is relevant to both of them.

This is a throwout bearing assembly from Chevrolet Performance. The braided steel line is the bleeder port, with the red cap hiding the bleed nipple fitting. You’ll read more about it below. (Image/Summit Racing)

Checking Your Hydraulic Clutch Fluid

As alluded to above, your brake system and hydraulic clutch system share many similar parts. So if you’re comfortable working around a brake system, this should all be pretty easy for you.

For starters, your hydraulic clutch will likely have a fluid reservoir that’s accessible from under the hood. Sometimes the reservoir is attached to a slave cylinder, but depending on the clutch actuator design, its common for the two components to be separate. It’s a good chance that the hydraulic clutch fluid reservoir will be near your brake’s master cylinder.

Here’s a typical hydraulic clutch fluid reservoir on a modern car. The cap is a simple twist-lock design, and the hose connects to the clutch slave cylinder about a foot below the reservoir. Do you see how dark and dirty this fluid is? That’s your clue that it needs to be changed. (Image/OnAllCylinders – Paul Sakalas)

Remove the cap. You’re checking for two things. First off, check to make sure your fluid level is in spec. If it’s too low, you’ll risk sucking air into the system and that’ll cause a serious loss in hydraulic pressure.

Next, examine the quality/color of the fluid itself. Normally, the fluid is clear with perhaps a yellowish tint. But if you haven’t changed it in a while, there’s a good chance it’ll be cloudy, dirty, and dark. If that’s the case, you should purge/bleed the system to replace the old fluid.

If you’re fighting a squishy clutch pedal and your fluid looks clean (or…even if it doesn’t) you may have an air bubble in your hydraulic line. In a lot of cases, a simple bleeding can fix the issue. Keep reading for details on that process.

Changing Hydraulic Clutch Fluid

The good news is, the process of changing your hydraulic clutch fluid is dang-near identical to bleeding brakes. Better still, you don’t have to deal with multiple lines going to each caliper/wheel cylinder, so the process is probably going to be a whole lot easier than a brake job.

To begin, you’re going to have follow the fluid line from the reservoir/slave cylinder down into the engine/transmission.

We’ll speak in general terms because front-wheel transaxle vehicles have a slighty different look than a rear-wheel drive setup. And based on the type of clutch actuator you have (pushrod or bearing, explained above), your bleeding job will be slightly different too.

Where the hydraulic clutch line meets the engine/transmission, you’ll find one of two things: either a spot on the transmission with a bleeder nipple, or a separate braided line with a bleeder valve dangling from the assembly. (That’s because of the different clutch actuator designs we talked about earlier.)

Either way, crack the bleeder nipple and fluid will (hopefully) start dripping out. While you can gravity-bleed the system in a lot of situations, if you’re cycling-in new fluid, it’ll be a lot faster to use a vacuum bleeder tool to suck the fluid through the line. With the vacuum tool attached on the nipple, start pulling the old fluid out, while keeping a keen eye at the reservoir in the engine bay, topping it off with new fluid to ensure it doesn’t run dry.

Not familiar with the brake bleeding process or specialized bleeder tools? Read this.

Once the fluid coming out of the bleeder valve starts looking clean (and bubble-free) in the vacuum hose, remove the vacuum tool. Now, if gravity is allowing fluid to drip out, you may be able to simply tighten the bleeder valve shut and be good to go.

But, in the author’s experience, it’s helpful to have a friend hop in the driver’s seat and, with the bleeder valve closed, have them press and hold the clutch pedal. From there, you crack open the bleeder valve one last time. The pressurized fluid squirts out (taking the last bit of bubbles and grit with it), and you close the valve. Then, your friend can release the clutch pedal, the system will be bled, and you’re on your way.

Important! Brake or hydraulic fluid can eat away at surface finishes, particularly paint. Make sure you wipe off any excess brake fluid quickly, and rinse the surface with water to prevent damage.

Here’s the hydraulic fluid line (circled, bottom) going into a FWD vehicle’s transaxle. Above the line’s banjo fitting is the (arrowed) bleeder valve port. (Image/OnAllCylinders – Paul Sakalas)

How Often Should You Check Your Hydraulic Clutch Fluid

While opinions differ, you’ll probably want to change your hydraulic fluid every time your change your brake fluid. As mentioned above, the two systems often share fluids, which makes it easier to shop for. And, it tends to be a messy, drippy job, so it’s probably best to get it done all at once.

So then question is, when should you change your brake fluid?

Check your brake fluid with every oil change, looking for things like dramatic level changes and color/tint of the fluid. Most vehicles can go two, three, or even four years without changing brake fluid. When your hydraulic fluid starts to look milky, dirty, or generally gross, that’s your cue to make the change.

Author: Paul Sakalas

Paul is the editor of OnAllCylinders. When he's not writing, you'll probably find him fixing oil leaks in a Jeep CJ-5 or roof leaks in an old Corvette ragtop. Thanks to a penchant for vintage Honda motorcycles, he spends the rest of his time fiddling with carburetors and cleaning chain lube off his left pant leg.