My 80 Dasher does the same thing....sound like the diaphram is gone away which 
is located inside the booster unit.

The check valve is located somewhere on the line running from the pump to the 
brake booster...

Make sure you have a good pump, that is drawing plenty of vacuum.  The 
diaphram within the pump sometimes goes bad. Start the engine, remove the line 
running into the booster power brake unit, you should feel plenty of draw.

If you do, then your only other alternative is to go get a used booster unit
or replace it with a new one.

I basically live with that problem on my 80 Dasher.  The pedal gets hard when 
we come up to a stop sign, but once the engine gets up its rev's, the boost 
goes back to normal
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         (o o)    "Betty Boop BBS"  Carmel Valley: Netgate off Highway 1
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