Can't find replacement o-ring for 1989 f-150 5.0 psps

every possibility
as the switch has to report oil pressure and if the seal or diaphragm is damaged then oil will leak out the switch

go to a auto parts store and request part --look online for it or if you have it go match it up at auto parts / hardware store

FIRST YOU DRAIN POWER STEERING PUMP USING A SIPHON. PLACE DRAIN PAN UNDER VECHICLE WHERE POWER STEERING PUMP LOCATION WOULD BE.THE DRAIN PAN WILL CATCH FLUID AS YOU REMOVE POWER STEERING LINE.YOU NEED FLARE IN LINE WRENCHES AND A NEW POWER STEERING PRESSURE LINE AND NEW O - RINGS. IF NEW PRESSURE LINE DONT HAVE NEW O- RING THE DEALERSHIP WILL HAVE IT.

I have no experience doing this conversion myself but that code should have nothing to do with the engine itself. The code indicates that you're having a problem with the power steering pressure switch (psps). I suspect that the connector to it got left of or damaged during the engine swap. The psps senses when the steering system is loaded, such as tight parking turns and then tells the computer to increase idle speed so that the engine does not stall. If the wire to the switch is left off or damaged, the code will set. You'll need to inspect the high pressure line coming from the power steering pump. Somewhere between the pump and the steering rack you will see a small oil pressure switch with a couple of wires going to it...or at least it should, the connector might be hanging somewhere near it.

SOUND LIKE POWER STEERING PRESSURE HOSE LEAKING.COPPER TUBING IS TOO SOFT METAL FOR POWER STEERING AND BRAKES SYSTEM.BUY A PRESSURE HOSE AT ANY AUTO PARTS PLACES LIKE AUTO ZONE OR ADVANCE AUTO PARTS. PRESSURE HOSE SHOULD HAVE O-RING COME WITH IT.

Hopfully this will be ALL you need Besides Pulley REMOVAL TOOL It can be Rented @ AutoZONE

REMOVAL

1. Remove drive belt.
2. Raise vehicle on hoist. WARNING: TURN AIR SUSPENSION SWITCH (LOCATED IN LUGGAGE COMPARTMENT) OFF TO PREVENT VEHICLE DAMAGE OR PERSONAL INJURY.
3. Remove power steering pressure hose bracket.
4. Disconnect electrical connector at power steering control valve actuator.
   1. Install Steering Pump Pulley Remover T69L-10300-B on the pulley hub. CAUTION: Do not apply in-and-out pressure on the power steering pump shaft. Pressure will damage the internal thrust areas.
   2. Hold the small hex-head and rotate the tool nut counterclockwise to remove the power steering pump pulley.
   3. Disconnect power steering pressure hose at power steering pump and drain fluid into a container.
   4. Disconnect power steering return hose at power steering pump and drain fluid into a container.
   5. Remove four retaining bolts and power steering pump and power steering pump pulley assembly.
   6. Remove power steering pressure switch from pump.
   INSTALLATION
   
   1. Position the power steering pump on the mounting bosses of engine block and install the bolts at the side of the power steering pump. Tighten to 20-30 Nm (15-22 ft. lbs.).
   2. Connect the power steering reservoir pump hose to the power steering pump and reposition the hose clamp to power steering pump inlet fitting.
   3. Replace power steering pressure hose O-ring.
   4. Install the power steering pressure hose to the power steering pump fitting. Tighten the tube nut with a tube nut wrench rather than with an open-end wrench. Tighten to 34-46 Nm (26-33 ft. lbs.). NOTE: Swivel or end play of the power steering pressure hose end is normal and does not indicate a loose fitting.
   5. Place the power steering pump pulley on the power steering pump shaft and install Power Steering Pump Pulley Replacer T91P-3A733-A. The small diameter threads on the tool should be engaged in the power steering pump shaft. CAUTION: Do not apply in and out pressure on the shaft. Pressure will damage the internal rotating group.
   6. Hold the small hex-head and rotate the tool nut clockwise to install the power steering pump pulley on the shaft. CAUTION: Do not apply in and out pressure on the shaft. Pressure will damage the internal rotating group. NOTE: The power steering pump pulley face must be flush within 0.25 mm (0.010 inch) of the end of the pump shaft.
   7. Remove the Power Steering Pump Pulley Replacer T91P-3A733-A.
   8. Remove the shipping tube nut from power steering pump housing prior to installation.
   9. Connect electrical connector to power steering control valve actuator.
   1. Install power steering pressure hose bracket.
   1. Lower the vehicle.
   2. If equipped, turn the air suspension ON.
   3. Install drive belt on the power steering pump pulley.
   4. Fill the power steering pump reservoir with Power Steering Fluid Motorcraft MERCON(r) ATF.

What are we talking about? Preasure side of AC compressor pump? Power steering pump?
If AC, then possibly from a leaking AC-high preasure switch on the back of the compressor. The leak could be simply the O-ring. You simply unscrew it and install the replacement making sure the O-ring is good and in place. The switch can leak also. This part is hard to find accept at the dealer. It has a 4-wire connector on it.

there is a teflon o ring seal that goes on the fittings, make sure its there...

I had the same problem. You can not replace that o ring. You have to replace the complete hose ***. They redesigned the hose to fix that common problem. To bad the customer has to pay to replace a poorly designed part.