During the exterior preflight inspection of a B757-200; a Captain noticed the brake wear pins nearly flush with the minimum indication plate on three wheels of the right main gear. Recent changes in his carrier's maintenance procedures resulted in the aircraft being flown with an undetermined percentage the braking effectiveness compromised.

Narrative

Safety problem induced by a change to aircraft maintenance procedures. During the exterior preflight inspection of our aircraft I observed brake wear pins flush or nearly flush with the minimum indication plate; on 3 wheels of the right main gear (#3 aft; #4 forward; and #4 aft). I immediately reported this to maintenance personnel. The subsequent replacement of the #3 aft and #4 forward brake assemblies and the deferral of the #4 aft assembly; required over 2 hours and resulted in a departure delay of 1 hour; 50 minutes. Maintenance personnel and Maintenance Control informed me that a recent change in our maintenance procedures removed brake inspections from the 70-hour service checks and that aircraft brakes are now inspected at 220-hour intervals. Our aircraft still had 46 hours remaining before the end of such an interval. It would appear that the recent change in maintenance procedures resulted in the current safety hazard; where an aircraft had flown with an undetermined percentage of its braking effectiveness compromised. While I have always included a check of the brake wear pins in my preflight inspections; I reviewed both our flight manual and the computer based training material; on the B757/B767 exterior inspection and found no specific mention of this check. Our flight manual; states only to check wheel and tire condition.This event raises questions: What criteria are being used as a basis for the change in the interval of brake inspections? Is there a feedback loop in place? Is this the only aircraft which had flown with brakes worn beyond serviceable limits; or are other aircraft in our fleet flying with worn out brakes?