A restricted vacuum circuit may eventually reach the expected pressure but take too long to get there. This distinction matters: a stronger pump will not correct a closed valve, undersized passage, blocked filter, or high-resistance manifold.
Long small-bore lines, abrupt fittings, contaminated filters, slow valves, narrow mold channels, and poorly connected cavities reduce conductance. Restrictions are especially damaging during the first fraction of the forming cycle, when the largest air volume must be removed.
Diagnosis
Pressure should be compared at two or more points during the same cycle. A substantial transient difference between the receiver and tool indicates flow resistance. The circuit should be checked in order from the mold face back to the receiver, including ports that appear visually open but lead into narrow internal passages.
Lines should be kept short and generously sized for the required flow, but no single diameter applies to every tool. Valve flow coefficient, total volume, target evacuation time, and pressure range must be considered. Cleaning or redesigning a restriction should be verified by a faster pressure curve, not only by a higher final gauge reading.