M20 pilot experiences a vacuum pump failure during an IFR flight. Flying partial panel; the reporter elects to divert to a VMC airport and lands safely.

Narrative

While flying in cruise; the wing leveling autopilot failed. The decision was made to cancel the IFR flight plan and divert VFR since the conditions at destination were 1;000 FT ceiling and the pilot preferred to not have to shoot the approach with a malfunctioning autopilot. The weather deteriorated so a climb was initiated; contacted Center; and requested an IFR pickup. It was then discovered that the vacuum pump had failed. So; flying partial panel; we requested an IFR clearance initially back to departure airport. Then; after the hand off to Center; we diverted to ZZZ and completed the ILS approach and landing with no issues. The problem arose from not immediately suspecting the vacuum system when the wing-leveler malfunctioned. The indications from the gyros were still good and the vacuum gauge was reading lower than normal. Flying in marginal VFR conditions took a high degree of concentration so it's not known when the vacuum gauge finally dropped to zero. The pilot had his last IPC in April and had flown 4 approaches within the last 2 months. To prevent a recurrence; the replacement vacuum pump will have wear indicator ports to track condition; and regardless of condition the pump will be replaced every 3 years or 500 hours; whichever comes first. The pilot will of course continue to practice partial panel and maintain IFR currency as keeping these skills up to date were vital in the successful outcome of this event.