On March 31, 1999, about 1943 Pacific standard time, a Fokker F.28 MK 4000, N477AU, registered to Atlantic Credit Corporation, and operated by Horizon Air as a 14 CFR Part 121 scheduled passenger flight was en route to Seattle, Washington, when the primary hydraulic system lost pressure. The flight diverted to Spokane, Washington, where a landing was made without further incident. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft was not damaged. The two airline transport pilots, two cabin crew and 40 passengers were not injured. The flight had departed from Billings, Montana, as Horizon Flight 2419, about one hour and 15 minutes prior to the incident. The flight crew reported that, while in cruise flight, the number 1 hydraulic (utility) fluid quantity dropped below the low red mark on the gauge. The flight crew notified maintenance control, and a decision was made to divert to Spokane. The secondary hydraulic system was utilized for the remainder of the flight to Spokane. The flight crew reported that this was the second time that day that the number 1 hydraulic system fluid level had dropped. While in Billings, after the previous occurrence, maintenance personnel serviced the system with hydraulic fluid. The flight crew then ran the engines and cycled the flight controls while maintenance personnel looked for leaks. No leaks were found and flight 2419 departed for Seattle. The aircraft was inspected by Horizon Air maintenance personnel in Spokane, who reported that a line for the flap drive motor was found chafed through and leaking in the left side wheel well. Maintenance personnel removed the chafed line and installed a flex line in accordance with the F28 maintenance manual instructions.