PLT OF SMT ACFT DEVIATED FROM ASSIGNED TRACK AND OVERSHOT CLB ALT DUE TO AN ACFT EQUIP PROBLEM.

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WE DEPARTED ADDISON ARPT WITH OUR IFR RELEASE. RWY HDG 2000 FT; WE CONTACTED DEP 124.3 AND CLRED TO 4000 FT WITH 360 DEG HDG. DURING THE CLB; WE NOTICED THE DIRECTIONAL GYROSCOPE WAS TURNING VERY SLOWLY AND WHILE TRYING TO DETERMINE OUR HDG; WE INADVERTENTLY CLBED TO 4200 FT INDICATED ON OUR ALTIMETER. THE CTLR ASKED FOR OUR HDG AND ALT. WE THEN REALIZED OUR HDG WAS 030 DEGS ON THE COMPASS. WE RPTED ALT 4200 FT AND HDG 030 DEGS. THE CTLR REPEATED 'ALT OF 4000 FT AND HDG 360 DEGS.' WE APOLOGIZED AND STATED THAT WE WOULD CORRECT OUR ERROR; WE IMMEDIATELY DSNDED TO 4000 FT AND ATTEMPTED TO TURN L TO 360 DEGS USING OUR COMPASS. AS WE STARTED THE L TURN; THE DIRECTIONAL GYROSCOPE BEGAN SPINNING ERRATICALLY. AT THIS TIME WE REALIZED THE DIRECTIONAL GYROSCOPE WAS TOTALLY UNRELIABLE. WE WERE ALSO UNSURE OF OUR ATTITUDE INDICATOR. THE INST PRESSURE GAUGE WENT DOWN TO 3 1/2 BUT THE LOW PRESSURE WARNING INDICATOR SHOWED NORMAL. WE ALSO INSTINCTIVELY CHKED ALL CIRCUIT BREAKERS AND ALL WERE NORMAL. WE REALIZED CONTINUED FLT WAS OUT OF THE QUESTION AND RPTED THAT WE WERE EXPERIENCING DIFFICULTY WITH THE DIRECTIONAL GYROSCOPE AND REQUESTED RADAR VECTORS BACK TO ADDISON ARPT. THE CTLR SAID 'UNDERSTAND YOU WANT RADAR VECTORS BACK TO ADDISON.' WE ANSWERED 'YES SIR; THAT IS AFFIRMATIVE.' HE THEN SAID 'DO YOU NEED ANY ASSISTANCE?' WE REPLIED; 'NOT RIGHT NOW SIR.' CTLR SAID 'TURN TO 170 DEGS AND DSND TO 3000 FT.' BECAUSE THE COMPASS WAS SWINGING DUE TO TURB IT TOOK SEVERAL SECONDS TO STABILIZE EVERYTHING BEFORE BEGINNING OUR TURN. THE DIRECTIONAL GYROSCOPE APPEARED TO STABILIZE AND WE RESET WITH THE MAGNETIC COMPASS. AS THE CTLR DID NOT SPECIFY THE DIRECTION OF TURN; WE BEGAN A L TURN TO 170 DEGS AND DSNDED TO 3000 FT. ONCE STABILIZED AT 3000 FT AND 170 DEGS THE CTLR TOLD US TO TURN TO 180 DEGS. AFTER OUR TURN TO 180 DEGS; WE CONTACTED APCH AND STATED OUR HDG AND ALT IN ORDER TO CONFIRM THE ACCURACY OF OUR INSTS. WHEN THE CTLR RESPONDED; WE NOTICED THEY HAD CHANGED CTLRS AS THE VOICE WAS QUITE DIFFERENT. THE CTLR CONFIRMED OUR HDG AND ALT AND TOLD US TO TURN TO 200 DEGS. AFTER A WHILE HE GAVE US A HDG OF 270 DEGS; THEN TO 300 DEGS UNTIL ESTABLISHED ON THE LOC 33 FOR ADDISON. DURING THIS TIME THE DIRECTIONAL GYROSCOPE APPEARED TO BE WORKING. WE CONTINUED TO XCHK WITH THE COMPASS. WE DSNDED ON THE LOC UNTIL WE HAD VISUAL WITH THE RWY; THEN CONTINUED VISUALLY TO LAND. AFTER WE ARRIVED AT OUR BASE; WE WERE ADVISED THAT DFW APCH WANTED US TO CONTACT THEM. LATER THAT EVENING; WE CONTACTED THEM. FINALLY; THE REASON WE DID NOT DECLARE AN EMER AT THE TIME WAS THAT WE WERE ABLE TO CONTINUE WITH COMPASS AND TURN COORDINATOR WHILE TRYING TO DETERMINE THE PROBLEM; WHICH BEGAN TO IMPROVE. UPON RETURN TO OUR BASE; THE MECHS FOUND THAT ONE OF THE VACUUM PUMPS HAD FAILED.