Unfortunately it's pretty common for the EXPENSIVE Hydraulic Control Unit to fail in that generation of Passat. The valves stick and or solder joints break. Some people have had success with opening the unit, removing the conformal coating, then reflowing solder (a co-worker of mine got lucky this way). It seems like most have ended up either replacing the HCU or retrofiting some valves from a Ford (Mustang, I believe) ABS pump into the VW unit. I remember seeing some threads on a different forum for this about a year ago....
-Rod