Dry air pumps are extremely vulnerable to contamination, particularly by liquids. The graphite vanes are designed to operate absolutely dry, and the introduction of any liquid can quickly destroy a pump. One of the most common causes of premature dry pump failure is contamination by solvents used to wash down the engine compartment after maintenance. Even a small amount of this stuff can cause the brittle graphite pump vanes to fracture in short order. Another common cause of pump failure is oil contamination.
Oil can enter the vacuum pump in several ways.
One frequently-seen culprit is a leaky pad seal gasket between the pump flange and the engine accessory case. Actually, any engine compartment oil leak that allows oil to get on the pump may find its way inside through the drive coupling. Alternatively, oil that gets on the vacuum regulator will quickly oil-soak the foam garter filter and start being sucked inside the pump itself. If even a tiny bit of oil gets inside a dry pump, it's history. A dry pump can also be destroyed by carbon contamination.