Golf IV / Jetta IV :: 1998 - Clutch Engages Lower To The Floor / Shifter Stuck In Gear

98 Jetta TDI hydraulic system ... We just received this vehicle from my mother in law, who is not driving any more. It's a cream puff--79,000 miles, beautiful condition. Problem is the clutch engages lower and lower to the floor, and eventually the shifter gets stuck in gear. The brake pedal slowly sinks to the floor when at stops on hills. Our mechanic tells us these two are connected because both the clutch and brake run off the same hydraulic system.

1st time he said the fluid was filthy, and he emptied the system and put in clean fluid. Worked for a little while and then we were back where we started. Second time he replaced a couple parts on the clutch, replaced this fluid, and this time it ran like a dream for a little over a month. Then the problem just returned. Again he says the fluid is filthy, and now we are looking at replacing the master cylinder and again changing the fluid. The theory behind why this is happening is that my mother in law let the car sit for half a year before passing it on to us. She didn't do much driving the 6 months before that either. I've done research, and this doesn't appear to be a common problem by any means on these cars.