1999 Chevrolet Cavalier clutch reservoir cap

Do you mean you can't find it on the car or you can't find a replacement? There is no cap. You buy the master cylinder, slave cylinder, and line as an assembly already filled with fluid.

Yup. You buy the entire system as a package. GM has always been famous for making you buy more than you need by building everything into assemblies, like their HEI distributors, generators with built-in voltage regulators, etc, but Ford handles their hydraulic clutch systems the same way. Sometimes there are aftermarket fittings you can buy at an auto parts store, but you have to destroy the original ones to get the steel or plastic lines out. None of the hose connections are meant to come apart.

If you are able to find replacement fittings, the only way you'll get just the reservoir is to buy a used one from a salvage yard. Even if they were to sell you just that one part, they're going to charge you the same as if you bought the complete package, because taking just the reservoir makes the rest of the parts worthless to the next potential buyer.

If the hydraulic part of the system is working okay so you don't need a new master cylinder, head to a salvage yard if all you need is the cap for the reservoir. Normally the caps can't be removed, but if yours can, that's your best bet. Keep in mind that most clutch systems use brake fluid and that is not compatible with any type of petroleum product like engine oil, transmission fluid, power steering fluid, or grease. We don't even like to get our fingerprint grease on the rubber bladder seal under the reservoir cap. One drop of oil in the brake fluid will make all the rubber seals and parts swell up and get mushy. The only fix for that is to replace everything in the system that has rubber seals or other parts that contact the brake fluid, and to flush and dry the steel lines. With one drop of oil in the brake system that can render an older car not worth repair.

If you find a reservoir cap that has been removed already and has been getting rained on, water won't hurt anything once it is dried off. Water in the brake hydraulic system will cause one type of brake fade when it boils and turns to vapor. Brake fluid can easily exceed the boiling temperature of water, but that isn't a concern in a hydraulic clutch system. The only concern there is that moisture will promote corrosion of the metal parts.