THE PILOT MADE A PRECAUTIONARY LANDING TO SECURE THE CABIN DOOR. AFTER LANDING, THE AIRPLANE GROUNDLOOPED DURING A HIGH SPEED RIGHT TURN OFF THE RUNWAY ONTO THE TAXIWAY. THE RIGHT PILOT'S LEFT BRAKE PEDAL WAS 'SPONGY', WHILE THE OTHER BRAKES SEEMED NORMAL. AFTER THE ACCIDENT, AIR WAS BLED FROM THE LEFT BRAKE CYLINDER. ON THE PREVIOUS DAY, THE FLOATS HAD BEEN REMOVED FROM THE AIRPLANE AND WHEELS REINSTALLED. THIS WAS THE PILOT'S FIRST LANDING WITH THE AIRPLANE USING THE RIGHT SET OF BRAKES AFTER REINSTALLATION OF THE WHEELS.