The usual answer to this is, forever. Of course, as is the case with many hard and fast answers, there are always exceptions.
The reason that your car clutch fluid should last forever is that it’s a closed system. It consists of a reservoir, a master cylinder, a slave cylinder and some tubing, and as long as nothing goes wrong within the system, low clutch fluid should never be a problem.
How your clutch works
When you push the clutch pedal, the fluid is forced from the reservoir into the slave cylinder, which then applies force to a throw-out bearing. This causes the clutch to disengage from the flywheel. As long as there’s enough fluid in the cylinder, everything works as it should, and you won’t need to top off or change the fluid.
Problems resulting from leaking clutch fluid
If you have an older vehicle, some of the components could become compromised, and small leaks could develop. You don’t need to be alarmed if you have to top off the fluid once in awhile, but if it gets to the point that the clutch no longer works the way it should, then you’ll have to have some repairs made, and at that point, the fluid will have to be replaced.
Usually, when you’re leaking too much fluid the problem is the slave cylinder. It’s easier to replace if it’s outside the bell housing. In newer vehicles it’s usually inside the housing, and that means the transmission has to be taken out in order to do the repair, which is not typically something mobile mechanics can do. Fortunately, with a newer vehicle, this isn’t a problem that’s likely to occur unless you keep the vehicle for a very long time.
If your fluid needs topping off, you can do it yourself. There is actually no such thing as clutch fluid – brake fluid is always used, so pick up some DOT-3 or DOT-4 brake fluid. The difference in the two is that DOT-4 has a higher boiling point, but the kind you’ll use depends on your vehicle. Your owner’s manual should tell you which kind to use, but if you’ve lost the manual or it doesn’t specify, one of our mechanics can recommend the type that will work best for your vehicle. The reservoir is usually near the rear of the motor compartment, and the mechanic will pour in the brake fluid using a funnel.
If topping off doesn’t correct the problem, though, one of our mechanics can diagnose and repair the problem.