AFTER ACCEPTING A CLB CLRNC TO MAINTAIN VISUAL SEPARATION WITH CONFLICTING TFC; AN ACR FLIES A TCASII RA.

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ON CLB WITH ZBW WE WERE CLRED TO CLB AND MAINTAIN 15000 FT. IN CLB AT APPROX 12000-13000 FT; WE WERE ASKED BY ZBW IF WE HAD AN ACR X; AN MD80; IN SIGHT AT 3 O'CLOCK POS; 16000 FT AND 5(?) MI. WE SEARCHED FOR AND SPOTTED THE MD80. WE TOLD ZBW WE HAD THE MD80 IN SIGHT. ZBW THEN CLRED US TO CLB TO FL350 WHILE MAINTAINING SEPARATION FROM THE MD80. WE ACCEPTED THE CLRNC WHILE WATCHING THE MD80. THE FO WAS FLYING THE ACFT WHILE I WATCHED THE MD80 XCOCKPIT THROUGH THE FO'S SIDE WINDOW. IT APPEARED THE MD80 WOULD CONVERGE WITH OUR FLT PATH BUT SLIGHTLY BEHIND US. JUST AS I WAS ABOUT TO TELL FO TO LEVEL AT 15000 FT; WE RECEIVED A TCASII TA; FOLLOWED ALMOST IMMEDIATELY BY AN RA. THE RA DIRECTED US NOT TO CLB. I TOLD FO TO LEVEL OFF AND HE DID AT APPROX 15100-15200 FT. (I DO NOT KNOW IF ACR X MD80 RECEIVED AN RA OR NOT.) I WILL NOT ACCEPT A VISUAL CLB AGAIN UNLESS I AM CERTAIN -- BOTH VISUALLY AND FROM ATC -- THAT POSSIBLE TFC CONFLICTS ARE ACTUALLY DIVERGING FROM OUR FLT PATH. IN THIS CASE WE WERE WATCHING THE OTHER ACFT AND LEVELED OFF AND WERE ARRESTED BY TCASII. HOWEVER; I DO NOT WANT TO RELY UPON VISUAL JUDGEMENT AND TCASII AGAIN AT THE SPDS WE ARE TRAVELING IN CLB AND CRUISE!