manual brakes???
MacDaddy,
When I had my 81 Malibu cop car with a built 69 350 I had the same problem..
.513 lift cam with a bunch of duration, 1.7 roller rockers, etc.
I solved the problem simply and cheaply with a vacuum reservoir....$ 35 thru Summit
http://www.summitracing.com/parts/SUM-G1464/