GM-GMC: Clutch will not disengage, clutch master cylinder, chevy s10 pickup

Question
I wrote you yesterday about a clutch problem on my friend's Chevy S10 pickup.I really goofed up on explaining the problem. It is a 1992 model S10 4 wheel drive.. All at once the clutch will not disengage. No previous problem.When the engine is started it takes off, even with clutch depressed or gear shift in neutral position. When the motor is not running you can change gears and even put it reverse.Any suggestion on where to start the trouble shooting?

Answer
All that you are describing points to the clutch hydraulic system, except for the part about the vehicle taking off WITH THE GEAR SHIFT IN NEUTRAL.  If that is the case, and I have never seen a vehicle do that, there is something wrong with the shifter mechanism or there is an internal transmission problem.
If it is not true that the vehicle moves under engine power with the shifter in neutral, there are a number of possible failures.  Either, or both of, the clutch master cylinder and slave cylinder are bad, there is a leak in the system and the fluid is low, or there are loose or missing parts where the slave cylinder engages the clutch throwout bearing arm.
Being able to shift into different gears with the engine off is quite normal.