B757 ACFT ON TKOF AND CLB THE #2 ENG EPR REMAINED LOW AND THE THROTTLE WOULD NOT ADVANCE TO FULL TRAVEL. FLC DISCUSSED THE PROB; CALLED THEIR COMPANY MAINT AND IT WAS DETERMINED TO DIVERT TO A CLOSE ARPT FOR REPAIR. POSTFLT INSPECTION FOUND A CIRCUIT BREAKER PULLED AND COLLARED AS A RESULT OF PREVIOUS MAINT WHICH WAS THE CAUSE OF THE ENG FAULT.

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A NORMAL PREFLT WAS COMPLETED. THE LOGBOOK REVIEW SHOWED ONE DEFERRED ITEM HAD BEEN CLRED BY OAK MAINT THE PREVIOUS EVENING -- A PROB WITH THE L REVERSER. THE MAINT RELEASE APPEARED COMPLETE AND ACCEPTABLE. TAXI AND PREPARATION FOR TKOF WERE NORMAL. THE FO WAS THE PF. WHEN CLRED FOR TKOF; HE STOOD THE THROTTLES UP; WAITED FOR ENGS TO STABILIZE; ADVANCED THROTTLES TOWARD R-TO PWR; AND CALLED FOR ME TO PUSH THE EPR SWITCH TO ENGAGE THE AUTOTHROTTLE. PWR SETTINGS FOR THE REDUCED PWR TKOF WERE 1.33 EPR AND A XREF OF 83 PERCENT N1. THE AUTOTHROTTLE MOVED THE THRUST ON THE L ENG TO 1.32 EPR AND BTWN 81-82 PERCENT N1. I CHKED THE EPR AND N1 ON BOTH ENGS; CHKED THE AIRSPD ON BOTH SIDES AND ANNOUNCED 90 KTS. I TOOK A QUICK GLANCE OUTSIDE AND THEN RECHKED THE ENG GAUGES. THE L WAS STILL SLIGHTLY LOW BUT WITHIN NORMAL LIMITS FOR AUTOTHROTTLE SETTING AT 1.32 EPR AND NOW ABOUT 82 PERCENT N1. INSTEAD OF JUST LIGHTLY MONITORING THE AUTOTHROTTLE AND GUARDING THE THROTTLES; I ATTEMPTED TO ACTIVELY 'TWEAK' THE L THROTTLE. THE THROTTLE MOVED VERY SLIGHTLY FORWARD AND HIT A MECHANICAL STOP APPROX 2 - 2 1/2 INCHES SHORT OF FULL TRAVEL. THE ACFT WAS NOW WELL INTO THE HIGH SPD REGIME OF THE TKOF AND ACCELERATION WAS NORMAL. GIVEN THOSE 2 FACTORS AND THE PREDEP BRIEFING THAT SHORT OF AN ENG FAILURE; FIRE; OR REVERSER UNLOCKED; WE WILL CONTINUE ANY TKOF FROM HIGH SPD UNLESS IT SEEMS THAT THE ACFT WILL NOT FLY; I ELECTED TO CONTINUE THE TKOF. OUT OF 3000 FT FOLLOWING THE AFTER TKOF CHKLIST; I BEGAN TO MORE ACTIVELY ATTEMPT TO IDENT THE CAUSE OF THE PROB. I CHKED THE REVERSE LEVEL DOWN; CHKED FOR ANY CIRCUIT BREAKERS POPPED; AND RETARDED THE THROTTLE SLIGHTLY AND ATTEMPTED TO PUSH IT FORWARD AGAIN. IT HIT THE SAME MECHANICAL STOP. INTERESTINGLY; DURING THE CLB AS THE CLB EPR INCREASED; SO DID THE ACTUAL INDICATED EPR REMAINING ABOUT 0.01 BELOW WHAT WAS CALLED FOR BY THE THRUST MGMNT COMPUTER. I TOLD THE FO I THOUGHT WE HAD A PROB. HE CHKED THE SAME THINGS I DID AND WE BEGAN TO DISCUSS OUR OPTIONS. HE EXPRESSED CONCERN ABOUT SINGLE ALT DRIFTDOWN OVER THE MOUNTAINS IF THE R ENG SHOULD QUIT ENRTE. I OFFERED THAT WE MIGHT USE A B737 TECHNIQUE OF HAVING DRIFTDOWN ALTERNATES FOR EACH SEGMENT OF THE FLT IF THE WX WAS ACCEPTABLE. WE CALLED DISPATCH AND BEGAN TO DISCUSS OUR PROB AND REQUESTED INFO ABOUT THE WX AT ACCEPTABLE ENRTE ALTERNATES. I BECAME CONCERNED; NOT ABOUT THE DRIFTDOWN AND LNDG; BUT ABOUT THE POSSIBILITY OF A SINGLE ENG GAR AT A HIGH ALT ARPT VERSUS A REDUCED PWR TKOF AT SEA LEVEL. WE ASKED DISPATCH TO CONNECT US WITH SYS ACFT MAINT CTL. WE DISCUSSED OUR PROB AND ASKED HIS OPINION ABOUT WHAT THRUST WE COULD EXPECT FROM THE L ENG GIVEN OUR SIT. HE STATED WHAT WE ALREADY EXPECTED; THAT THERE WAS NO WAY TO KNOW. HE RECOMMENDED THAT WE RETURN TO THE BAY AREA FOR AN ENG CHK. WE CONCURRED AND ADVISED DISPATCH THAT WITH THEIR BLESSING WE WOULD RETURN TO SFO IN ORDER TO HAVE BETTER PAX OPTIONS AND BETTER PARTS AVAILABILITY. WE LANDED NORMALLY AT SFO. SUBSEQUENT MAINT INSPECTION OF THE ENG REVEALED THAT IN WORKING ON THE REVERSER THE EVENING BEFORE; THE MECH HAD PULLED A CIRCUIT BREAKER IN THE COCKPIT AND SAFETIED IT WITH AN ORANGE COLLAR. A COLLARED BREAKER TO A PLT INDICATES THAT MAINT HAS TAKEN APPROPRIATE ACTION AND IS NOT A CONCERN; ESPECIALLY GIVEN A CLEAN MAINT RELEASE; NO WARNING LIGHTS; AND NO EICAS WARNINGS; ADVISORIES; OR CAUTIONS. MY RECOMMENDED SOLUTION IS FIRST; ENGINEERING IDENT ALL CIRCUIT BREAKERS THAT WOULD POTENTIALLY POSE THIS KIND OF PROB -- THAT IS; PULLING THE CIRCUIT BREAKER COULD ADVERSELY AFFECT THE FLT BUT GIVE NO CREW WARNING. SECOND; MAINT SHOULD BE REQUIRED TO POST AN ORANGE OR RED 'NO FLY' PLACARD ON THE CTL WHEEL OR STICK ANY TIME ONE OF THESE CIRCUIT BREAKERS IS USED AS A SAFETY FOR THEIR WORK UNTIL THE CIRCUIT BREAKERS HAVE BEEN RESET.