On October 1, 1997, about 1615 eastern daylight time, a Grumman G-73, N168WS, registered to Pantechnicon Aviation Ltd., experienced a collapse of the nose landing gear during the landing roll at the Fort Lauderdale-Hollywood International Airport, Fort Lauderdale, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 type-rating flight. The airplane sustained minor damage and the airline-transport rated type rating applicant/pilot, second pilot, and an FAA operations inspector were not injured. The flight originated about 1215 from the same airport. The pilot-in-command stated that the gear operated normally during the flight which included several gear extensions and retractions. While on final approach, the landing gear indicator indicated that the gear was down which was confirmed once after landing gear extension, and again on short final. After touchdown during the landing roll at an indicated airspeed of about 40 knots, the nose landing gear collapsed. Post incident trouble shooting of the hydraulic system revealed that the landing gear hydraulic selector valve failed to hold pressure resulting in the collapse of the nose landing gear. Pressure testing of the valve revealed that a poppet valve part number 27580, was bypassing fluid. The O-rings were replaced and the poppet valve continued to bypass fluid.