fuel trouble help

my Buddys got a 01 F350 with a 7.3 I think. It has a loss of power and the service engine soon light came on. He changed out the fuel filtter with no change. The filtter looked like it had been sucked dry. We have also cleaned out the fuel filtter bowl and flushed out the fuel lines. It still runs like it is starving for fuel. We seen alot of air coming thru the fuel inlet tube but that has cleared up of the most part. (we put 5 gal. of fuel in it) We have also found ALOT! of pressure when opening the peck cock to drain the fuel out of the bowl and when I mean ALOT I mean it. It is at the point now where it is blowing the seal out on the fuel filtter cap. My ideal is that maybe the fuel return line is blocked or the fuel pressure regulater is bad. any help would be gerat.