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. The Miata owner doesnt realize that anything has happened until one day the clutch does not work. Thats how I found out about it anyway. After I refilled the empty fluid reservoir (it uses brake fluid), the clutch came back to life, but I knew eventually I would have to replace that slave cylinder. I ordered one from Roebuck Mazda for about $48. You can get a rebuild kit for less, but it won't fix it for long if there is any wear inside the piston cylinder. Easier and almost as cheap is to just replace it. That said, you can go for a long time without replacing it, its a VERY slow leak. Just keep the reservoir topped up. The replacement job is easily done by the do-it-yourselfer with a little preparation and the right tools. To replace it, jack up the front right side of the car and support it with a stand. Remove the wheel. The slave cylinder is sitting right there in front of you inside the opening in the wheel well. Under the hood, get as much fluid out of the clutch fluid reservoir (not the brakes!) as you can, because when you disconnect the hydraulic line from the cylinder, its all gonna run out. You can try to stop it, but it will 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). 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 for your ratchet socket wrench, now you can unbolt that sucker while sitting comfortably on your butt with your head in the wheel well. Now get the old one off and bolt on the new one, and make sure to put some grease on the end of the piston thingy so it doesn't make noise (while you're driving). Now reconnect the hydraulic line, careful now, its real easy to get that guy cross-threaded and then you're really screwed. So just make it finger tight to start with. Then tighten it down with a wrench, but don't use gorilla torque, it doesn't need it. Now you just gotta bleed the hydraulic line and you are done...
Heres how to bleed the clutch hydraulic system, the old fashioned way. Get some clear tubing about 0.25 inch inside diameter, maybe a little smaller, even (this is a Home Depot item). 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. 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. You need an 8 mm wrench to open the bleed valve. Turn it only about a half-turn. 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 (takes a lot of running around the car). When the pedal feel firms up and it looks like clean fluid is coming out, you are probably done. This is the one-person method that I just made up as I went along, it's a lot easier with another person to help. Also, when you do the slave cylinder, 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 that sucker out with a paper towel. Mine had about a quarter inch of muck in the bottom. Believe me, if I can do this, anyone can, I am no mechanic.
Total time to complete job, about 1 hour and that includes time to clean up all the brake fluid I got on the paint (don't ask). Now the clutch pedal feel is much firmer and the clutch engages a little higher in the pedal travel (no more air in the line).
The slave cylinder is a pretty common issue with Miatas. I had to replace mine as soon as I got the car. 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 boot and see if fluid comes out. The boot can hold the fluid and not appear to be leaking. Changing out the slave cylinder is not very hard or expensive. I got a new Bendix Slave Cylinder from Advanced Auto for 35 USD and the install only took about 45 minutes. Now that I know how I can do it in less time I'm sure. Bendix Slave PN is 12966
Note: Always use a flair nut wrench on the brake line fitting. If you don't there is a good chance you will damage the nut.