I have a 1998 400 series model # 28V707 yard machine lawn mower that the belts came off. I thought that I put the belts back on right but when I start the mower it takes off like it is in gear and the Clutch /Brake won't stop it

Sure about that model number, here's what MTD has to say about that MTD Products . That looks more like a Briggs engine number rather than the tractor number