Brake pressure stabilizer??

[Anthony Attard <ACA@nrc.gov>]

Good morning Audi Lovers.  I have a problem that I hope you
can help me with. 
I have a 1986 Audi CS 4000 Quattro.  Every now and then,
when I apply the brakes, the pedal goes to the floor.
Normally the pedal is solid as it should be. This is totally
random behavior.  I have replaced the "brake pressure
stabilizer" (I think this what it's called),  a couple of
times already. Is this a typical problem with the 4KQs ??,  
Is there anything I can to do to fix this permanently??
 Thanks everyone.  I truly appreciate your feedback.

Tony Attard

81 5K Diesel
83 TQC
86 4kCSQ
89 90Q