The pilot reported that during the landing roll a 'rumbling noise' was heard and the airplane pulled slightly to the left when he applied the brakes. He stated he pulled back on the elevator to no avail, and the airplane nosed down and flipped inverted. Post accident inspection revealed the front wheel on the left float separated from the airplane. The wheel attachment bracket then dug into the sod airstrip which resulted in the airplane nosing over. Metallurgical examination revealed the left caster axis bolt failed due to a fatigue fracture.