FLC OF A B727 OVERSHOT ASSIGNED ALT ON DSCNT DUE TO FAILING TO LEVEL AT ASSIGNED ALT.

Narrative

WHILE DSNDING ON THE MODESTO 2 ARR TO SFO WE WERE CLRED TO 'DSND AND MAINTAIN FL240 AND EXPECT TO CROSS CEDES AT 11000 FT AND 250 KTS.' WE DSNDED BELOW FL240 TO APPROX FL233 BEFORE THE ERROR WAS NOTICED AND WE RETURNED TO FL240. WE WERE NOT AWARE OF ANY CONFLICTS AND ZOA NEVER CALLED US OR COMMENTED ON THE ALT ERROR. WE WERE SUBSEQUENTLY CLRED LOWER AND THE REST OF THE FLT WAS NORMAL. THE MAIN CONTRIBUTING FACTORS TO THIS EXCURSION WERE: 1) OUR AIRLINE HAS 2 DIFFERENT TYPES OF AUTOPLTS AND ALT ALERTING SYS. ONE HAS AN AURAL TONE; A 1000 FT TO GO; AND AUTOLEVELS IF SELECTED; THE OTHER WILL NOT AUTO LEVEL AND HAS A LIGHT WITH NO TONE. OUR ACFT THIS DAY HAD NO AUTO LEVEL OR TONE ALERT. 2) MY ATTN WAS DIVERTED APCHING LEVELOFF BY A LARGE ERROR IN THE OMEGA COURSE AND MY EFFORT TO DISCONNECT THE OMEGA AND CORRECT TO COURSE. 3) MY DSCNT CALCULATION SHOWED ME HIGH FOR THE EXPECTED XING RESTR AND I WAS RECALCULATING MY DSCNT. 4) I AM A LOW TIME PLT IN THE B727 WITH LESS THAN 60 HRS SINCE QUALIFYING; AND MY PREVIOUS ACFT HAD THE AUTO LEVEL AUTOPLT FEATURE. I WAS STILL AWARE OF THESE TRAPS AND LET MYSELF BE DISTRACTED AT THE TIME I NEEDED TO BE LEVELING OFF. STANDARDIZING THE AUTOPLTS AND ALT ALERTING SYS WOULD HAVE HELPED AVOIDING THIS PROB.