73 duster brake question
Just an FYI, understand that vacuum is the "suction" that is found in the intake manifold while the engine is running. The power booster has a rubber diaphram in side that is actuated by the suction from the intake manifold.
It is not an electrical thing, so has nothing to do with ignition, fuel injection computers or anything else.
Big, long duration cams work with low engine vacuum, especially when the throttle is applied. If you don't also have a reserve vacuum cannister or electric vacuum pump, if the engine dies you will probably only have power assist for the first pump of the pedal. After that it will be a real problem to modulate your braking.
Disc pad type/ compound also make a big difference. Street pads have good low temperature friction function. High speed or racing type pads work best when the pads are hot. SSo the first stop with a hard pad will not stop well but as they heat up they work better.