What is the most likely cause of a steady but consistently low vacuum gauge reading during a manifold vacuum test?

A steady but consistently low vacuum gauge reading during a manifold vacuum test is most indicative of a vacuum leak in the intake system. When there is a leak, outside air enters the engine's intake manifold, disrupting the normal vacuum levels that the engine should produce during operation. This erratic condition leads to a lower reading on the vacuum gauge because the engine struggles to create the necessary vacuum for optimal combustion and intake flow.

While leaking valves, broken valve springs, and a blown head gasket can all cause variability or different types of problems in engine performance, they typically manifest with more erratic or fluctuating vacuum readings rather than a steady low vacuum. A vacuum leak results in a steady low reading due to continuous uncontrollable intake of air, which reduces the overall vacuum level, making it the most likely explanation in this scenario.