B767. FLT CREW RELEASED THE BRAKES AFTER THEY WERE CHOCKED AT THE GATE. THE ACFT ROLLED BACK 8 FT AFTER THE CHOCKS DID NOT HOLD ON AN ICY RAMP.

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PARKING WAS NORMAL AT GATE IN MODERATE SNOW WITH A MARSHALLER AND 2 WINGMEN. GOT CHOCKS-IN SIGNAL. I RELEASED THE BRAKES AND WE RAN THE PARKING CHK. A MIN OR SO LATER; I NOTICED THE JETWAY MOVING. ON LOOKING MORE CLOSELY; IT WAS THE ACFT ROLLING BACKWARDS; NOT THE JETWAY; WHICH WAS MOVING. I IMMEDIATELY APPLIED BRAKES AS RAPIDLY AS POSSIBLE; MINDFUL OF EVERYONE STANDING UP IN THE CABIN. LOOKING FORWARD; I THEN SAW A FRANTIC MARSHALLER PICKING HIS FLASHLIGHT/WANDS UP OUT OF THE SNOW TO GIVE ME A SET BRAKES SIGNAL. HE CAME ONTO THE HEADSET AND SAID THAT WE HAD ROLLED BACK ABOUT 8 FT IN THE SNOW AND ICE; THE CHOCKS HAVING SIMPLY BEEN PUSHED ALONG; UNABLE TO HOLD THE AIRPLANE ON THE SLOPING RAMP. HE SAID THAT JUST WALKING ON THE ICE AND SNOW WAS EXTREMELY HAZARDOUS. HE WAS VERY EXCITED; AS HE SAID THAT HIS FOOT WAS NEARLY RUN OVER; BUT I HAD STOPPED JUST IN TIME. NO ONE WAS INJURED ON THE GND OR IN THE AIRPLANE. I LEFT THE BRAKES SET BEFORE DEPARTING THE ACFT. I COULD FIND NOTHING IN OUR MANUALS GIVING NON-STANDARD GUIDANCE FOR THIS OCCURRENCE. TALKING TO MY BROTHERS WHO ARE WDB CAPTS AT ACR Y AND ACR Z; I LEARNED THAT THEIR FLT CREWS NEVER RELEASE THE BRAKES AFTER PARKING -- IT IS ALWAYS LEFT TO THE MECHS. I BELIEVE WE SHOULD CONSIDER MODIFYING OUR PARKING PROCS IN ICE OR SNOW. I WILL NEVER RELEASE THE BRAKES AGAIN UNDER SUCH CONDITIONS.