Replacing the Slave Cylinder

A very common problem on older Miatas is a leaky clutch slave cylinder. The seals inside the cylinder get old and your hydraulic fluid starts to slowly leak out.

Symptoms of a bad slave cylinder are low or empty clutch fluid. If the fluid looks very black, that can also be a good indication it's bad. One way to be sure is to pinch the rubber boot on the slave and see if fluid leaks out. The boot can hold the fluid and not appear to be obviously leaking until you squeeze. The Miata owner sometimes doesn't realize that anything it wrong until one day the transmission will not go into gear, or you may notice the engine "pulling" when you have the clutch fully depressed. If you're lucky, you may be able to refill the empty fluid reservoir (it uses brake fluid), and the clutch may come back to life, but eventually you will have to replace that slave cylinder.

A replacement is widely available from internet vendors, your local Mazda dealer, or your Friendly Local Auto Parts Store (FLAPS). A successful repair can also be made by rebuilding the cylinder. It just has a spring and a few rubber parts that are easily replaceable. As long as the bore of the cylinder is in good shape, the repair should last.

The replacement job is easily done by the do-it-yourselfer with a little preparation and the right tools.

To replace it:
  1. Jack up the front right side of the car and support it with a stand.
  2. Remove the right front wheel. The slave cylinder is sitting right there in front of you inside the opening in the wheel well.
  3. Under the hood, get as much fluid out of the clutch fluid reservoir as you can, because when you disconnect the hydraulic line from the cylinder, it’s all gonna run out. You will need a 10 mm wrench for the hydraulic line fitting, and a 12 mm for the bottom bolt of the slave cylinder (which you must access from under the car).
  4. The top bolt holding the cylinder on looks real easy to get to, and it is, but you can't turn a wrench in the small area that the slave cylinder occupies. That's why you need at least a 10 inch extension with a 12mm socket wrench, now you can unbolt that sucker while sitting comfortably on your butt with your head in the wheel well.
  5. Remove the old cylinder and bolt on the new cylinder, and make sure to put some grease on the end of the piston ram so it doesn't make noise.
  6. Now carefully reconnect the hydraulic line. It is usually easier to get the hydraulic fitting threaded if you do it before you firmly bolt down the slave cylinder. Be careful not to cross-thread!

Here’s how to bleed the clutch hydraulic system, the old fashioned way.
  1. Get some clear tubing - about 0.25 inch inside diameter, maybe a little smaller, even (this is a Home Depot item).
  2. Then take a clear jar or bottle and pour about 2 inches of brake/clutch fluid in it. You'll probably need a couple 12 ounce bottles of DOT 3 or 4 brake fluid, so you can flush a bunch of it through the system.
  3. Stick one end of the tubing on the bleed valve and the other end in the fluid in the bottle. That will keep air from getting sucked back up into the tubing.
  4. You need an 8 mm wrench to open the bleed valve. Use a flare wrench if you have one - it will be less likely to round off the fitting.
  5. Turn the wrench only about a half-turn.
  6. Then just start stepping on the clutch while keeping the reservoir filled, and checking the tubing every few pumps to see what's coming out. This is a lot easier with a helper.
  7. When the pedal feel firms up and it looks like clean fluid is coming out with no more bubbles, you are probably done.
  8. Also, while all the fluid is out of the system, pull the reservoir out of the master cylinder (it will pop right out if you pull hard enough) and wipe it out with a paper towel. Be careful not to get any brake fluid on your paint!

Total time to complete job, about 1 hour.