WHILE IN A PARKING LOT, THERE WAS AN ALMOST COMPLETE LOSS OF BRAKES. THE BRAKE PEDAL WOULD NOT DEPRESS. ONLY AFTER PRESSING MULTIPLE TIMES AND VERY HARD WAS THERE ANY BRAKING ACTION WHICH SLOWED THE TRUCK. GM HAS ISSUED A SERVICE BULLETIN, PIT5361 ABOUT THIS ISSUE. THIS IS A KNOWN ISSUE.