B727-200 CREW HAD A GPWS 'TERRAIN; TERRAIN' WARNING ON APCH TO ONT.

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ON A VECTOR TO RWY 26L ONT AT 3200 FT MSL; AUTOPLT ENGAGED; ALT HOLD ON. WE WERE ORIGINALLY GIVEN A 320 DEG HDG FOR INTERCEPT; THEN GIVEN 340 DEG HDG INTERCEPT. WX WAS CLR; 7 MI AT ARPT; A HAZE LAYER WAS PRESENT IN THE VALLEY. WE COULD VISUALLY SEE ALL TERRAIN AROUND US. AS WE PASSED OVER A SMALL RANGE WE RECEIVED A GPWS 'TERRAIN; TERRAIN' WARNING. I EXECUTED AN ESCAPE MANEUVER PER FLT HANDBOOK. THE AURAL WARNING ONLY WENT OFF ONCE AND THEN STOPPED; WE CLBED 500 FT TO 3700 FT MSL. RIGHT AFTER THIS THE CTLR GAVE US A 250 DEG HDG TO JOIN FINAL. THE FO TOLD HIM WE RECEIVED A GPWS AND HAD EXECUTED AN ESCAPE MANEUVER. AS WE ROLLED OUT ON 250 DEG HDG; I VISUALLY PICKED UP THE RWY. THE CTLR THEN SAID 'CLRED ILS RWY 26L -- BY THE WAY YOU ARE INSIDE THE MARKER.' WE TOLD HIM WE WOULD EXPECT A VISUAL APCH AND SWITCHED TO TWR. ACFT WAS STABILIZED APCH CRITERIA AT 800 FT AGL. NORMAL LNDG. ON THE GND; WE REQUESTED PHONE NUMBER TO TRACON AND I SPOKE TO MGR. HE SAID THEY WOULD INVESTIGATE. I HAVE 2 QUESTIONS: 1) WHO DID OUR GPWS GO OFF? 2) HOW CAN WE GET AN ILS APCH CLRNC INSIDE THE MARKER WITH FULL SCALE GS DEFLECTION? MY FRONT END CREW WAS OUTSTANDING IN MAINTAINING SITUATIONAL AWARENESS DURING EVENT.