MOONEY M20J PLT HAS LOSS OF THE ATTITUDE INDICATOR (AI) WHILE IN IMC.

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THE ACFT ENTERED INST CONDITIONS UPON DEP FROM GAITHERSBURG; MD; ON AN IFR FLT TO LYNCHBURG; VA. SHORTLY AFTER THE ACFT CAME LEVEL AT 6000 FT MSL; SOMETHING WENT WRONG WITH THE ATTITUDE INDICATOR (AI). THE GYRO TUMBLED VERTICALLY; LIKE AN EYEBALL ROTATING UPWARD INTO A PERSON'S HEAD. BASED ON TROUBLESHOOTING AFTER THE FLT; THE ANOMALY APPARENTLY WAS CAUSED BY PARTIAL LOSS OF PRESSURE IN THE ACFT'S VACUUM SYS. AN AVIONICS TECHNICIAN SAID HE FOUND LEAKAGE IN A HOSE GOING TO BOTH THE ATTITUDE INDICATOR AND THE DIRECTIONAL GYRO. THE LOSS APPARENTLY WAS NOT ENOUGH TO TRIGGER THE 'LO VACUUM' WARNING LIGHT ON THE ANNUNCIATOR PANEL; WHICH DID NOT ILLUMINATE. (THE LIGHT HAD WORKED OK IN THE PREFLT CHK BEFORE DEP.) THE DIRECTIONAL GYRO CONTINUED TO FUNCTION NORMALLY; DESPITE THE PROB WITH THE ATTITUDE INDICATOR. AT THE TIME; THE FLT WAS CLRED DIRECT TO THE MARTINSBURG VOR; WHICH WOULD HAVE BEEN A MAGNETIC COURSE OF APPROX 280 DEGS. THERE ARE SOME GAPS IN MY KNOWLEDGE AND UNDERSTANDING OF THE ENSUING DEVELOPMENTS; BUT I AM ABLE TO RPT THE FOLLOWING: I IGNORED THE EXTREME NOSE-DOWN INDICATION ON THE ATTITUDE INDICATOR; WHICH WAS SO INCONGRUOUS AS TO BE AN OBVIOUS INST PROB. I STARTED COMPARING THE L-R BANK INDICATIONS ON THE ATTITUDE INDICATOR AGAINST THE TURN COORDINATOR; FINDING THAT THEY STILL SEEMED TO AGREE. I SHIFTED MY FOCUS TO THE TURN COORDINATOR; IN AN EFFORT TO KEEP THE AIRPLANE UPRIGHT; BUT SOMEHOW ALLOWED THE PLANE TO DRIFT OFF COURSE TO THE R. JUST AS I NOTICED THAT THE HDG ON THE DIRECTIONAL GYRO WAS SOME 90 DEGS TO THE R; ATC CALLED AND SAID WE WERE TRACKING N INSTEAD OF W. AROUND THE SAME TIME; I NOTICED THE ALTIMETER UNWINDING RAPIDLY. WE LOST 400 FT BEFORE I WAS ABLE TO ARREST THE DSCNT AT 5600 FT. I DO NOT KNOW HOW I FIRST DETECTED THAT THE ACFT WAS DSNDING. I DON'T KNOW IF IT WAS THE CHANGING ALTIMETER READING OR PERHAPS THE SOUND OF THE ENG. I DID NOT TAKE NOTE OF EITHER THE AIRSPD INDICATOR OR THE VERT SPD INDICATOR. I ALSO DO NOT KNOW HOW I REALIZED THAT THE SITUATION REQUIRED A CORRECTION TO THE L -- WHETHER THAT WAS FROM READING THE ATTITUDE INDICATOR OR TURN COORDINATOR OR SIMPLY FROM KNOWING THAT THE TRACK HAD DEVIATED TO THE R. LET ME JUST SAY THAT I AM GRATEFUL FOR TRAINING RECEIVED FROM THE MOONEY ACFT SAFETY FOUNDATION IN THE RECOGNITION OF AND RECOVERY FROM AN INCIPIENT SPIRAL. I CREDIT THIS TRAINING WITH ENABLING ME TO MAKE THE CORRECT CTL INPUTS TO REGAIN CTL OF THE AIRPLANE. I DID NOT DETECT ANY G FORCES EITHER DURING THE UNCTLED DSCNT OR DURING THE RECOVERY. WITH THE AIRPLANE BACK ON COURSE AND BACK AT ALT; I COVERED THE ATTITUDE INDICATOR WITH A RUBBER PATCH AND BEGAN TO SORT OUT OUR OPTIONS. I WOULD LIKE TO HAVE ENGAGED THE AUTOPLT AT THIS POINT; BUT BELIEVE THE AUTOPLT DEPENDS FOR GUIDANCE ON THE ATTITUDE INDICATOR AND THEREFORE WOULD HAVE BEEN UNAVAILABLE. LUCKILY; WE SOON BROKE OUT OF SOLID IMC AND FOUND OURSELVES BTWN LAYERS. LEAVING THE FREQ; WITH PERMISSION; I CALLED FLT SVC AND VERIFIED THAT WE WERE HEADING TOWARD BETTER CONDITIONS. THE LAYERS EVENTUALLY GAVE WAY TO CLR WX; AND THE FLT PROCEEDED UNEVENTFULLY TO THE DEST.