WITH ENGS SHUTDOWN PARKING BRAKE PRESSURE BLEEDS OFF AND ALLOWS ACFT TO ROLL BACK ON SLOPE.

Narrative

AFTER A NORMAL PUSHBACK FROM PIT INTL TERMINAL; WE TAXIED OUT FOR TKOF. WHILE TAXIING WE WERE GIVEN AN ATC DELAY FOR TKOF OF ABOUT 30 MINS. GND CTL THEN TOLD US TO PULL INTO THE RWY 28R RUN-UP PAD. AT THE PAD I THEN SET THE PARKING BRAKE AND SHUT DOWN THE #2 ENG. ENG #1 WAS NEVER STARTED AFTER PUSHBACK. WE LEFT THE APU RUNNING TO PROVIDE ELECTRIC PWR AND AIRCONDITIONING. THE TIME WAS XA05 WHEN WE SHUT DOWN #2 ENG. WE BOTH COMMENTED ON NOT HOLDING AT THIS SPOT IN YRS. WE ALSO DISCUSSED THIS SHARP 'DROP OFF' RIGHT BEHIND THE ACFT. AFTER PARKING OUR NOSE WAS FACING THE TXWY AND OUR TAIL POINT DIRECTLY AT THE APPROX 50 FT 'DROP-OFF' BEHIND US. IN PREVIOUS YRS I WAS A FO ON BOEING 737-200 ACFT AND WE HAD PARKED AT THIS POINT FOR ATC DELAYS WITH ENGS SHUT DOWN AND EXPERIENCED NO PROBS. THE HOLDING PAD IS INCLINED DOWN TOWARD THE DROP-OFF! THEN AT ABOUT XA20 LCL TIME MY FO THOUGHT HE HEARD THE PARKING BRAKE RELEASED BUT LOOKED OVER AT ME AND SAW THAT MY FEET WERE ON THE FLOOR AND MY SEAT WAS PULLED BACK FROM A NORMAL FLT POS. SO HE DID NOT SAY ANYTHING TO ME BECAUSE HE THOUGHT THE NOISE MAY HAVE COME FROM THE GALLEY. A FEW SECONDS ALTER I JUST HAPPENED TO LOOK OUTSIDE. TO MY SHOCK WE WERE ROLLING BACKWARDS. I THEN SLID FORWARD AND STEPPED ON THE TOE BRAKES BUT NOTHING HAPPENED - NO BRAKES AND WE WERE STILL ROLLING BACKWARDS TOWARD THE STEEP DROP-OFF. I THEN GRABBED THE EMER BRAKES AND THE ACFT STOPPED. THE FO THEN TURNED ON THE ELECTRIC HYD PUMPS (WHICH ARE NORMALLY OFF) AND BUILT UP THE HYD PRESS. I DID NOT REALIZE THAT THE BRAKES WOULD RELEASE BY THEMSELVES EVEN THOUGH THE ACFT HAS A BRAKE ACCUMULATOR! THIS IS A VERY DANGEROUS SIT BECAUSE OF THIS BLEED DOWN PROB IN THE F-28'S. I NOW KNOW THAT OTHER PLTS HAVE HAD THE EXACT SAME THING HAPPEN TO THEM. I SUGGEST A NEW MANDATORY PROC BE ADOPTED; IF ENGS ARE SHUT DOWN AND WHEELS ARE NOT CHOCKED; ELECTRIC BOOST PUMPS MUST BE LEFT ON!! OTHERWISE A VERY SERIOUS ACCIDENT WILL HAPPEN WHEN SOMEONE ROLLS OFF A PARKING SPOT WITH AN F-28 THAT HAS IT'S PARKING BRAKE 'SET.'