2004 Craftsman 54 inch 27 HP 700 Hours. Will start quickly - will idle and run up to max RPM - when engaging the PTO, most of the time it will stall - sometimes it will run with PTO engaged for a few seconds to a minute or two only. Installed new fuel filter-no change. When running, fuel filter is empty or nearly empty, but fills up when engine is off for a while. Fuel line between tank and filter is clear and fuel will run freely out of fuel line if not connected to filter. Could this be the fuel pump?