A B757-200 REPORTS OF ROUGHNESS FELT IN THE RUDDER PEDALS ON TAXI-OUT; INADEQUATE MAINT TROUBLESHOOTING AND COMPANY MAINT CONTROL WILLINGNESS TO DISPATCH AN ACFT WITHOUT KNOWING WHAT THE PROBLEM WAS.

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DURING FLT CTL CHK (ON TAXI OUT); I NOTICED ROUGHNESS IN THE RUDDER PEDAL. FO FELT IT ALSO. RETURNED TO GATE. THE LCL MECH FELT THE ROUGHNESS AND SAID HE SUSPECTED A BAD BEARING IN ONE OF THE PULLEYS FOR THE RUDDER CABLE. AFTER SPENDING SOME TIME LOOKING FOR THE PROB; THE PROB WAS NOT LOCATED; BUT SOME PULLEYS WERE FOUND TO BE IN GOOD CONDITION. MAINT CTL DECIDED TO FERRY THE ACFT BACK TO ZZZ. I DISCUSSED THE PROB WITH SEVERAL PEOPLE -- LCL MECHS AND 2 PEOPLE IN MAINT CTL. THEY HAD NOT FOUND THE CAUSE; BUT MAINT CTL INSISTED THE ACFT WAS PROBABLY SAFE TO FLY. I OBJECTED THAT WITHOUT KNOWING THE CAUSE OF THE PROB; I COULD NOT BE SURE THE PROB WOULD NOT GET WORSE. I DISCUSSED THE PROB WITH THE FO AND DECIDED TO REFUSE TO FLY THE PLANE. I FELT THE PRELIMINARY DIAGNOSIS WAS PROBABLY CORRECT; BUT IT WAS NEVER CONFIRMED. IT WAS ONLY A WELL EDUCATED GUESS. I BELIEVE THE PROB SHOULD BE FIXED; OR AT LEAST POSITIVELY IDENTED; BEFORE DISPATCHING A BROKEN ACFT. IN THE INTEREST OF EFFICIENCY; MAINT CTL WANTED TO ASSUME THE ACFT WAS SAFE ENOUGH TO FLY. IT IS SIMPLY NOT GOOD ENOUGH TO ASSUME THAT A PROB WITH FLT CTLS WILL NOT GET WORSE. I WAS LATER TOLD THAT SYS SCHEDULING WAS LOOKING FOR ANOTHER CREW TO FLY THE ACFT. I DO NOT KNOW IF SOMEONE FLEW IT. I DO NOT KNOW WHAT THE PROB ACTUALLY TURNED OUT TO BE. NEVER MAKE ASSUMPTIONS ABOUT MECHANICAL ISSUES OR ANY OTHER SAFETY OF FLT ISSUE.