In the good old days, a good brake job would have included bleeding some brake fluid out at the wheel cylinders to insure that no air had gotten into the brake lines. Then new fluid added to keep the master cylinder up to level would have freshened the fluid in the system eliminating the need for a total system flush. Now days it's just slap on new pads and rotors and send you out the door, so a flush at 100,000 - 150,000 miles would probably be a good idea IMHO. In 47 years of car and truck ownership I've had one brake line go out at 170,000 mi--the dealer recommendation at 70K was over kill.