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NA NB NC

NA-NB-NC Clutch Hydraulics

Original Author: Phil Barnett (prb) - 2013

Back to Maintenance

The clutch system in every Miata is hydraulic. This means there is not a purely mechanical connection between the clutch pedal and the clutch actuator arm. Instead, the mechanical transfer takes place through a hydraulic system.

The parts of this system are:
  • Clutch Pedal
  • Clutch Pedal to Master Cylinder link rod
  • Master Cylinder
  • Hydraulic line
  • Slave Cylinder
  • Slave cylinder to Clutch arm link rod.
  • Clutch Arm
  • Hydraulic Fluid
Generally speaking, there is no adjustment in this system. You can adjust the free play in the clutch pedal to master cylinder link rod. It's important that you retain the approximately 1/2" of free play from this section. If you remove the free play, the master cylinder cannot reset properly.

The two parts that commonly fail in the clutch system are the master cylinder and the slave cylinder. Generally, a slave failure exhibits fluid loss. Master cylinder failures may or may not exhibit loss of fluid.

Troubleshooting Steps

The first step of troubleshooting is to check the fluid level. To do this, you must remove the lid on the clutch master cylinder reservoir. Hydraulic fluid changes color with time as it absorbs water from the atmosphere. Because of this, it can stain the reservoir. what may appear to be a full reservoir to the outside can be completely empty on the inside.
  1. Experiencing Fluid Loss
    • Determine if the slave is leaking. The slave is located on the left side of the engine while standing in front of the car with the hood up. It is under the starter and is difficult to see from above. Generally, service of this is done from either the wheel well or from underneath. Getting the car up on ramps or jack stands is a big help. Once you have access to the slave cylinder, peel back the rubber boot from the slave cylinder. If there is any hydraulic fluid visible there, the slave is bad and must be replaced.
    • Inspect the hydraulic line that runs between the master cylinder and the slave cylinder. This line consists of a long metal tube leading to a flexible rubber portion, then terminated by a curly metal tube at the slave. Generally this line doesn't fail, but there have been reports of the rubber section going bad and becoming balloonlike. If this happens, it keeps the pressure from reaching the slave.
    • Inspect the master cylinder. If you have not found the fluid leak in the slave or the line, the master is leaking and needs to be replaced. Usually, master cylinders don't leak externally, but if they do, they leak inside the car near where the pushrod from the clutch pedal engages the cylinder. Peel back the carpeting and inspect the insulation in the area for signs of brake fluid.
  2. No Fluid Loss
    • You need to find out if the hydraulic system is working and the problem is elsewhere or if the master cylinder is bad. With an assistant in the driver seat while you watch the area of the slave cylinder, have them depress the pedal while you watch the clutch arm. With a full stroke of the pedal, the clutch arm should move over 1/2". If it does, the problem is likely inside the clutch area.
    • If the pedal depresses and the clutch arm doesn't move, the master cylinder is probably leaking around the front seal and will need to be replaced.

Replacement

With both hydraulic parts of this system going bad as often as they do, it's wise to replace both parts at once, both the master and the slave cylinders. While working on the hydraulic tubing in this system, you'll need some line or flare wrenches. These are specially designed wrenches that will protect the fittings. Using standard wrenches will cause damage to the fittings.
  1. Replacing the master
    • When replacing the master cylinder first measure the depth of the piston's hole (where the rod inserts) on the old one against the new one and then adjust the pedal engagement rod accordingly BEFORE starting to bleed the system. There can be as much as 3/8" difference in piston holes' depth between OEM and replacements.
    • Use a flare wrench to remove the hydraulic line from the front of the master cylinder.
    • Remove the nuts holding the master cylinder to the brake booster.
    • Slide the old master cylinder from the brake booster.
    • Bench bleed the replacement master cylinder
    • Mount the new master cylinder to the brake booster and torque the nuts.
    • Thread the flare nut and line into the master cylinder and tighten with a flare wrench.
  2. Replacing the slave
    • Remove the line from the slave using a flare wrench.
    • Remove the two mount bolts
    • Slide the slave forward and release the pin from the clutch arm.
    • Reverse the process for the new slave

Bleeding

Great, you've gotten the diagnostics and the replacement of the bad part completed.Now you need to make sure that all the air is out of the system. This process is called bleeding. Regardless of how you accomplish this, the result is to move fluid all the way through the system, pushing the air ahead of it. Once you have forced fluid all the way through the system, the air is pushed out and all that remains is hydraulic fluid.

In every Miata, the hydraulic clutch system is filled with Dot 3 Brake fluid. Dot 3 brake fluid, by design, is hygroscopic. What that means is that Dot 3 absorbs water, even from the air in the form of humidity. As it does this, it turns colors from clear to yellow and eventually to brown. At some point, the fluid has absorbed all it can and the result is that corrosion begins inside the system. Corrosion results in component failure. The hydraulic cylinders are sealed by rubber cups working against the sealing surface of a mirror polished cylinder. If corrosion begins on the mirror surface, the corrosion forms an uneven surface like sandpaper. The abrasive corrosion wears out the rubber cups and the seal is broken. Fluid leaks. The system stops pumping properly.

This is the primary reason that failure happens in hydraulic systems. Purely from lack of maintenance. It is recommended that you completely flush all hydraulic systems, both the clutch and the brakes, every two years. This pushes out the contaminated fluid and replaces it with fresh fluid that is not full of water. Corrosion is inhibited and the rubber cups can last many years. Remember this once you get your system back working again. Your have this choice. replace the fluid or replace the bad components it creates and also replace the fluid. Obviously it's less expensive in the long run to keep up with these fluid flushes.

There are 5 ways to flush out the air and old fluid. You'll have to choose one of them to complete the system bleeding. The are listed in ease of use, effectiveness and speed. Always begin by removing any old fluid from the reservoir and wiping the interior with a cleaning cloth. It's important to not allow the used brake fluid to go into the ground, Always attach a clear tube to the bleeder valve and run it into a catch container. Dispose of used fluids properly. A clear tube makes it easy to see when the old fluids and air are all out of the system and only clean brake fluid is exiting. The end of a couple of feet of 3/16" inside diameter tubing can be forced over most bleeders
  1. Power Bleeder
    • You attach a power bleeder to the master cylinder, fill it with fresh fluid, pressurize it and open the bleeder. The pressurized system forces fresh fluid through the system. Watch for clear fluid exiting the clear hose. Tighten the bleeder. Done. The advantage of this system is that it's extremely fast, can be done by one person and you don't have to worry about the master cylinder running low because it's being refreshed from the power bleeder as fast is it's being used. The cons are that this system is relatively expensive.
  2. Compressed Air Bleeding
    • Attach a clear hose to the bleeder nipple, lead it into a catch container and open the bleeder. Fill the master cylinder with fluid and replace the cap. Pump the clutch pedal twice to pull the first fluid into the master cylinder. Hold a blow tip near the cap vent to pressurize the system. As you do this, you will see the fluid level go down rapidly. Stop using compressed air when the reservoir gets low, fill reservoir, replace cap and repeat 4-5 times. After the 4th time, look at the clear line to see if the new fluid has reached the catch container. Tighten the bleeder.
  3. Vacuum Bleeding
    • This requires two people, one to keep the master cylinder reservoir full and one to do the work below the car. Begin by filling the master cylinder with fresh fluid. Remove the bleeder valve and wrap the thread with plumbers Teflon tape. Replace bleeder in threaded slave using the Teflon to seal the threads. Attach the vacuum gun to the bleeder with a clear tube and a catch container. Pump the vacuum up to draw fluid through the system. Have your assistant insure that the master cylinder never goes empty. When all old fluid and air has been pulled from the system and only clear fluid is coming from the nipple, stop the vacuum and tighten the bleeder.
  4. Manual Pump Bleeding
    • This requires two people, even better with three. One to pump the pedal, one to keep the master cylinder full and on to open and close the bleeder valve. It works like this. Fill the master cylinder reservoir with fresh fluid. Insure it doesn't go empty. With one person positioned near the bleeder, clear tube and catch container, open the bleeder. Have you assistant in the car depress the pedal. Close the bleeder. Have you assistant lift their foot. Open the valve, repeat. When all the old fluid and air have exited and you only see clear fluid escaping, close the bleeder.
  5. Gravity Bleeding
    • Fill the master cylinder reservoir with fresh fluid. Attach the clear hose leading to the catch container. Open the bleeder. Wait. Gravity will pull fluid through the system. Insure that the reservoir never goes dry. Some systems have a cap for the bottle where you suspend the bottle over the reservoir and it acts like a water cooler jug, glugging and filling the reservoir as the fluid is drawn through by gravity. This is a slow process and generally won't work if there is air in the lines since air tries to rise through the system against the flow of gravity.

Of all methods, it's imperative that you never allow the master cylinder reservoir to go empty because that will draw air into the system and you have to start over.

Replace your hydraulic fluid in your clutch and brakes every 2 years and your hydraulics will last for many years.