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Hello and thank you for your question,Your 1988 F 250 has built in check valves on each in tank fuel pump that are suppose to hold the pressure at 32 to 45 PSI on the first turn of the key and if its bleeding down after it sits for a while that usually means the check valves are failing. A quick way to check this is to try cycling the key on and off a few time before you actual crank the engine. If it then starts after doing so then the check valves are not holding and that means the pumps would have to replaced to resolve the problem.