PLT OF ATX SMA ACFT OPERATED BELOW MIN REQUIRED ALT DUE TO STAYING UNDER AN OVCST ENRTE TO DEST ARPT AFTER ATC HAD DSNDED THE ACFT TO VFR CONDITIONS.

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ENRTE FROM FSD TO OMA; I EXPERIENCED A VACUUM PUMP FAILURE WHILE IN IMC CONDITIONS AT 4000 FT MSL. I WAS IN CONTACT WITH OMAHA APCH AND EXPLAINED MY SITUATION. MY STATED INTENTIONS WERE TO USE THE ILS 32L APCH TO EPPLEY AIRFIELD; OR IF THAT DIDN'T WORK; A PAR 30 NO-GYROSCOPE APCH TO 30 AT OFFUTT AFB; THEN SVFR TO EPPLEY. AS MY GYROSCOPE INSTS FINALLY TUMBLED; I DECIDED TO FOREGO THE ILS AND THEN REQUESTED THE PAR 3C NO-GYROSCOPE TO OFFUTT. THE CTLRS WERE VERY HELPFUL AND I PROCEEDED THROUGH THE APCH WITHOUT ANY FURTHER PROBLEMS. UPON BREAKING OUT AND CALLING THE RWY IN SIGHT THE CTLR CLRED ME FOR THE OPTION. I HAD ALREADY STATED MY INTENTION OF SVFR TO EPPLEY SO I PROCEEDED AS PLANNED -- THAT WAS THE PROBLEM. THE WX HAD DETERIORATED TO WORSE THAN ADVERTISED (APPROX 500 SCATTERED WITH MORE LAYERS ABOVE THAT). I FOLLOWED THE RIVER (MISSOURI) TOWARDS EPPLEY. THE LOW CLOUDS FORCED ME TO FLY LOWER THAN I WANTED. I FLEW BTWN 300-400 FT AGL APPROX. THIS PUT ME UNCOMFORTABLY CLOSE TO THE VARIOUS BRIDGES; XING THE RIVER. I FINALLY MADE IT TO EPPLEY AND LANDED WITHOUT ANY FURTHER PROBLEMS. I FEEL RATHER STUPID FOR NOT LNDG AT OFFUTT WHEN I HAD THE CHANCE. IN RETROSPECT; I CAN SEE I PUSHED MYSELF TOO MUCH TO DELIVER MY CARGO. I CAN ALSO SEE I PUSHED THE RULES -- THE SVFR WAS OK; BUT I WAS TOO CLOSE TO THE GND. I THINK THE MECHANICAL PROBLEMS I DEALT WITH RATHER EASILY. THIS MAY HAVE BOOSTED MY CONFIDENCE FOR AN 'EASY' SHORT VFR HOP. IN THE FUTURE I WON'T PUSH MY LUCK. I'LL LAND WHILE I HAVE THE CHANCE.