A clutch master cylinder is used in vehicles with manual transmissions to pressurise the hydraulic fluid that presses and releases the clutch. When this component goes bad, it can make for some very difficult driving. There are several ways a clutch master can go bad, including air in the system, low fluid or internal failure.
Drive the vehicle, and check the free movement of the clutch. Anything more that 1/2 inch is excessive and a sign of a faulty clutch master cylinder.
Press the clutch hard to the floor and immediately release the clutch pedal. The pedal should quickly release and pop up to its starting position. If it slowly returns to the starting position or stays near the floor, the clutch master may be faulty or needs the air bled from it.
Check the fluid level in the clutch master cylinder. It should be between the low and high levels. Use the flashlight to see it if necessary.
Add fluid to the clutch master cylinder, and add UV dye to the oil by simply dumping it in.
Pump the clutch several times, then use the UV glasses and light to check for fluid leaks under and around the clutch master. Leaking UV dye is a clear sign that the clutch master cylinder is faulty and will require replacement.
Use only fluid indicated for your car--the majority will use DOT3.
Make certain you use the UV dye designed for oils--there are two types; one for coolant and another for oil.