[Vwdiesel] Brake SNAFU

[TexasTDI]

I have an interesting brake dilemma on my Jetta.  Since I first the car, the
brake pedal has always gone closer to the floor than I would like, and feels
a little squishy.  So I always pump the brake pedal when coming to a stop.
Pumping always made the brake pedal a little firmer.

Today, when I pumped the brake pedal, the pedal got so stiff it barely
moved.  Kinda like when you shut off the car and pump the brake pedal.
After 2 or 3 pumps the pedal won't move at all.  As an experiment I parked
the car in the driveway, with the engine running, and the E-brake off.  I
pumped the brake pedal and on the 2nd pump the pedal stiffened up and
wouldn't go down.  I then removed the cap off of the master cylinder and
tried it again.  Even with the cap off, the pedal would stiffen up after the
2nd pump.  And no fluid came out of the MS reservoir.

This has me completely stumped.  Anybody have any idea what could be wrong?
Master cylinder?  Brake booster?  Brake pressure regulator?

Christopher J. Thornton
1991 VW Jetta 2 Door 1.6 Diesel - 243k miles
2000 VW Golf 1.9 TDI (turbo diesel) - 111k miles
AMSOIL Synthetic Lubricants Dealer #1098500
http://www.amsoil.com