Welcome to the club. Very well documented issue. Many, many owners of that era Chevy and GMC trucks have had the rotted brake/fuel/transmission lines. Mine included. Many times the lines rot under the frame under the drivers side seat where the brake lines run into the ABS pump. When the mechanic checks, have them also look at the coolant lines that run from the engine to the radiator. Those are problematic as well. Was your truck ever in the "rust belt" states for a period of time. I ask because many of the GM fanboys who defend GM over these brake line issues claim that only trucks used in states that use salt or the melting spray solution on the roads in the winter are affected. I see you had the truck in Florida before moving to California.