My F-250 diesel 6.0L powerstroke is very hard to start after sitting for over 24 hours. If its used daily, there is no problem starting. A friend suggested to try a long crank (about 45 seconds), I tried until the oil pressure showed high on the display gage. I stopped and tried again, this time the oil pressure was up right away and finally the engine caught. I shut off the ignition and started right away. I suspected a low fuel pressure on the line as if the fuel was slowly draining and after 24 hours the fuel pressure was very low. What would cause this condition?