Power Assist Brake Vacuum problems of the 1997 GMC C3500

Brake booster malfunctioned, causing no braking ability which may have resulted in a crash. Also, driver and passenger rear seatbelts , were locked up, could not use the seatbelt and won't protect the occupant in event a crash. Dealer replaced both seatbelts. The rear of cab was not welded properly. Dealer had to weld the rear cab to front. Rear axle seal will go out, causing oil to go into the brake shoes, resulting in no rear brakes. The engine stalled. Had to get a rebuilt engine, and the manufacturer put out a TSB but has no solution. The engine fell apart just after warranty expired. The steering suddenly locked while the vehicle was making a turn.