Vehicles use hydraulic fluid for the brakes and clutch for manual transmissions. When checking hydraulic fluid, check the amount of fluid along, with the boiling point and amount of water it contains. These factors will determine how effective the fluid is, and if you need to change it.
Open the hood of the car, and locate the brake master cylinder. This will be mounted near the spot where the brake pedal would meet the firewall, and there will be a plastic container, the reservoir, mounted on top of it.
Open the cap on the master cylinder reservoir, and check the level of the fluid. There should be indicator marks on the reservoir, to show the minimum required level.
Siphon a small amount of fluid from the reservoir, using a turkey baster, and place a small drop into an optical refractometer.
Hold the refractometer up to a light to read the fluid moisture content and boiling point. You need to change it, if there is more than two percent water and/or a boiling temperature below 300 degrees Fahrenheit.
Siphon the remaining fluid out, if it needs changing. Abide by all local laws for disposing of hydraulic fluid.
Add new fluid to the cylinder and reservoir, as needed. Most vehicles require DOT 3 or 4 fluid. You can pour it straight from the bottle, but the baster will help prevent it from spilling.
Repeat the process for the clutch cylinder, if your vehicle uses manual transmission. If the fluid used for the brakes was contaminated, there is a good chance the fluid for the clutch is, as well.
