I have diesel toyota Innova purchased last yr

I have diesel toyota Innova purchased last yr. Unfortunately the car met with an accident fron the left side so the chassis had to be replaced.I got it replaced from a new showroom espirit toyota delhi. It was a recently opened showrom at that time. The car has done 13500kms now and i faced the problem of loose brake pedal (Pedal Dropping ) and i had to press quite hard till the extreme to stop the vehicle. I checked it at toyota showroom and they did the bleeding of all the 4 brakes as the hand brake also became loose. What could be the reason for this to occur and what precaution do i have to take in future so that i may not face problems regarding brake or higher expenses later on as my car is still under waranty so now i can get a part replaced.Please guide me