Two things spring to mind: The brake hoses, if original, could be expanding not unlike a balloon. If they are the originals, replace them, if only as a safety precaution. Pressure bleed the system after they are installed. Also, check that the rear brakes are properly adjusted. 

Karl

  ----- Original Message ----- 
  From: Victor Valenti 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Wednesday, September 29, 1999 2:30 AM
  Subject: Help, 84 Vanagon Brake Diagnosis

  I have a n 1984 Westy and have been having a braking problem. When the brakes are applied normally
  they feel fine, but when the brakes are stabbed hard (like in a panic stop) the peddle goes to the bottom of the stroke and you can feel it bottom in the master cylinder.  I have replaced the front and rear brake pads and shoes (drums turned) the master cylinder, the vacuum check valve and rear wheel cylinders the system has been bled manually, but still the problem persists. the only components left are the proportion valve and brake vacuum booster. The VW mechanics  I have talked to say that the vacuum boosters almost never go bad but it seems to me that it is the strongest suspect left.  Before I spend the big bucks and the major operation of taking apart the dash, I was wondering if anyone had encountered a similar problem or had some mechanical/hydraulic advice.
   
  Thanks 

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