It's not pulling enough vacuum because of the camshaft. Install a vacuum reservoir canister, that will fix your braking issue relatively cheap & easy.
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1972 SS 454 Chevelle LS5/M21/12 bolt 3.73 posi
1969 Malibu COPO L72/M20/12 bolt 3.31 posi
1968 Malibu LQ9/4L80E/12 bolt 3.73 posi
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