I have a 1999 Chevrolet Silverado 5.3 ltr engine with 112,000 miles. My fuel pressure gauge shows 59 lbs when the key is first turned on. After the pump shuts off after 1 1/2 - 2 seconds the fuel pressure drops to 0 immediately drops to 0. Is this normal or do I have a fuel leak somewhere. I have replaced the fuel filter, pump, heat sensor and fuel regulator.