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First of all what is low compression because everything before 1971 was a high compression 9.5:1 or better? Second 8 or 9 inch pound of vacuum is low you should be above 15 for the power brake to function. The timing could be too far retarded and this would cause low botton end power, low vacuum and sometime engine surge. What i was told to do is advance the ignition to achieve the highest steady vacuum and the motor does not ping under heavy acceleration. To check to see if the advance is working on the distributer or not put a timing light on the timing mark and remove vacuum from distributer you should see to timing change. You were also saying the vacuum advance when removed does not change the rpm of the motor this is because sometimes the vacuum is on ported vacuum on the carb not manifold vacuum.
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