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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