B767 ON UA324 AT MINDA INTXN; FL390 WAS OVERTAKEN BY A B747 ON DIRECT ROUTING TO MINDA FL390. SAW ACFT ON TCASII 4 MI IN TRAIL. CTR CTLR CLRED FLT TO DSND TO FL350. B747 ADVISED HAD THEM ON TCASII BUT DID NOT TAKE ANY ACTION.

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ENRTE FROM SAO PAULO; GRU; TO JFK ON AIRWAY UA324; LEAVING GEORGETOWN AIRSPACE; ENTERING PIARCO AIRSPACE; HAD BEEN ASSIGNED FL390 FOR PREVIOUS 50 MINS MAKING REGULAR POS RPTS AT FL390. WE WERE OVERTAKEN BY ACR AT FL390 IN A B747 DOING MACH .85. HE WAS ON A DIRECT ROUTING TO MINDA POS OFF AIRWAYS. HE CROSSED POS MINDA 8- 10 NM BEHIND US. WE SAW HIM POP UP ON TCASII JUST BEHIND US WITH SAME ALT. WE HAD JUST MADE MINDA RPT TO BOTH GEORGETOWN AND PIARCO WHEN PIARCO TOLD US TO DSND IMMEDIATELY TO FL350. B747 RPTED MINDA XA31Z FL390. WE HAD RPTED MINDA XA30Z FL390. WHEN B747 WAS QUESTIONED BY PIARCO CTL HE CONFIRMED THAT HE HAD SEEN US ON TCASII EARLIER. AMAZING THAT HE WATCHED US GET CLOSER AND DID NOTHING. OUR TCASII DID NOT PICK HIM UP FROM BEHIND UNTIL 8-10 MI. I BELIEVE TCASII DOES NOT DO WELL WHEN BEING OVERTAKEN. IN ANY EVENT WE NEVER GOT A TCASII TA OR RA. HIS SYMBOL STAYED WHITE. AS WE DSNDED THE DISTANCE CLOSED TO 4 MI. I QUESTIONED PIARCO CTL; HE SAID THAT GEORGETOWN HAD MADE A MISTAKE. HE ASKED US IF WE HAD BEEN IN CONTACT WITH GEORGETOWN. WE HAD BEEN IN GOOD RADIO CONTACT WITH GEORGETOWN AT ALL TIMES. I THEN SWITCHED BACK TO GEORGETOWN ON SECONDARY VHF RADIO AND ASKED ABOUT THE LOSS OF SEPARATION OVER MINDA. NORMALLY THEY MAINTAIN 10 MIN SEPARATION; NOT 10 MI. HE SAID THAT ACR B747 HAD BEEN CLRED DIRECT TO MINDA TO JOIN OUR AIRWAY. SOUNDED AS IF HE KNEW THAT HE HAD MADE A LARGE ERROR. SUBSEQUENT ATC SECTORS HAD PROBS KEEPING B747 IN RADIO CONTACT.