Normally there are 2 wear indicator pins on a brake, normally you would use the lowest pin for determining if the brake is within wear limits. It will be in the maintenance manual rather than the MEL. The manual will say you can dispatch with 1 missing/damaged provided the remaining one is checked prior to flight, a maintenance function rather than a pilot function. Also note that a wear pin even when flush still has sufficient capacity to allow for a full emergency stop. If outside of wear limits the MEL will state how many brakes can be inop and deactivated.