ALT OVERSHOOT AND BELOW GS DURING APCH TO PITTSBURGH; PA. FATIGUE IS A FACTOR.

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ON FINAL APCH FOR RWY 32 ILS AT PITTSBURGH INTL; THE CAPT DSNDED BELOW THE 4000 FT PUBLISHED STEP-DOWN ALT PRIOR TO THE STEP-DOWN POINT. I ADVISED HIM HE WAS LOW AND GOING BELOW GLIDE PATH. HE CORRECTED BACK TO THE GS. ABOUT THAT SAME TIME; APCH CTL ALSO ADVISED HIM HE WAS BELOW THE PUBLISHED ALT AT THAT DISTANCE FROM THE ARPT. HE ACKNOWLEDGED; REINTERCEPTED THE GLIDE PATH WELL PRIOR TO THE OM; AND COMPLETED A NORMAL FINAL APCH AND LNDG. WE WERE IN GOOD VMC CONDITIONS AT THE TIME; WITH THE ARPT; GND AND ACFT IN FRONT OF US (ABOUT 4 NM) IN SIGHT. I BELIEVE THERE WERE NUMEROUS CONTRIBUTING FACTORS FOR THIS SIT TO DEVELOP. THE FIRST FACTOR IS FATIGUE. THIS WAS THE LAST DAY OF A VERY LONG 4 DAY TRIP CHARACTERIZED BY LONG DUTY DAYS DUE TO WX DELAYS AND DEVS (TSTMS) FLYING COAST TO COAST EACH DAY; SFO-BWI-LAX-PIT. AS A RESULT; THE SCHEDULED 24+ HR TRIP ENDED UP BEING A 27 HRS 22 MIN TRIP. THIS LAST DAY OF THE TRIP STARTED AT XA15 IN LAX AND ENDED AT XH32 IN PIT. THE LAST LEG WAS SCHEDULED FOR 57 MINS; BUT WAS ACTUALLY 1 HR 43 MINS DUE TO TFC/WX DELAYS FOR TKOF AND LARGE ENRTE DEVS TO AVOID TSTMS AND CUMULO NIMBUS. THE SECOND FACTOR IN THIS SIT WAS THE NON STANDARD ARR DUE TO WX AND TFC ON THE ARR/DSCNT TO PIT. WE WERE FILED AND CLRED FOR THE NESTO ARR TO PIT; BUT PRIOR TO NESTO; WHILE WE WERE DSNDING TO 10000 FT MSL; APCH CTL VECTORED US TO INTERCEPT THE RWY 32 LOC. THIS PUT US ON FINAL APCH AT A HIGHER THAN DESIRED ALT. THE CAPT ELECTED TO SLOW AND CONFIGURE THE ACFT TO EXPEDITE OUR DSCNT TO THE PUBLISHED APCH ALTS. AFTER WE HAD SLOWED TO ABOUT 170 KTS; APCH CTL DIRECTED US TO INCREASE TO 210 KTS. THE CAPT COMPLIED AND WE AGAIN CHANGED THE ACFT CONFIGN TO MATCH THE INCREASED AIRSPD. THE CAPT WAS CONCERNED ABOUT CLOSING ON THE #1 ACFT ON FINAL (WE WERE #2) AND ALSO BEING TOO HIGH AND FAST AT THE OM. THIS IS WHEN HE INADVERTENTLY DSNDED BELOW THE 4000 FT PUBLISHED STEP-DOWN ALT. I BELIEVE HIS FATIGUE COMBINED WITH THE NON STANDARD; SOMEWHAT RUSHED APCH CAUSED HIM TO FOCUS ON HIS AIRSPD AND THE OTHER ACFT TO THE POINT THAT HIS ALT AWARENESS SUFFERED. FORTUNATELY; BOTH MYSELF AND THE APCH CTLR BACKED HIM UP AND CAUGHT THE PROB BEFORE IT BECAME MORE SERIOUS. THE SYS WORKED.