Q400 First Officer reports flight being canceled after FAA ramp inspection reveals brake wear pins nearing flush. Brake wear pin inspection is not a crew preflight item at this carrier.

Narrative

Flight arrived as scheduled. Mx and parts were brought along on the flight to fix an aircraft already broken. All was routine; our flight was boarding. An FAA representative came up to the flight deck and asked to see our certificates. As we were pulling them out he informed us that he'd be conducting a preflight and that he'd be checking the brake pad pin indicators. He stated that this was the reason the other aircraft was stuck here and that this was becoming an increasingly common issue. He came back 5-10 minutes later and stated that both of our pins were nearing flush. He stated that his impression was that they needed to be replaced at this time. We told him we would contact MX. MX checked the pins and decided to replace them. 20-30 minutes later it was decided to deplane the passengers. 45 minutes after that; the flight was canceled and we were told to go to the hotel.I don't know the MX inspection times; periods; or criteria for checking the brake pad indicators. It is not a required item on our preflight nor on our postflight. We are not given any written guidelines on the subject. I do not know the exact wear amount that is allowed; nearing flush sounds about right for replacement. I didn't get to see the brake pad indicators before they were replaced. Since it was the FAA bringing up the issue it's possible the brake pad indicators were within limits and MX erred on the conservative side; but I wasn't in direct contact with them. I believe the primary cause for the predicament was the lack of awareness on MX's side and the absence of the item in the preflight/postflight checklists. I don't see why this item isn't in our manuals. I know of several airlines that have this as a preflight check. It would have saved us at least 2 canceled flights; misplaced crewmembers; and unhappy passengers.