You said you replaced the fuel lines but the symptoms suggest it's still leaking air from somewhere. I know the M812 has a single passenger side fuel tank but the symptoms are familiar with an M818 that lived in my yard for quite some time. In this case, it was the spider hoses on the fuel selector valve. Once again, your truck does not have that but you are likely chasing an air leak. You stated that you changed your fuel filter. I would wonder if I got the o-ring seated correctly on the canister, whether the rectangular washer is seated correctly in the top of the housing and the condition of the o-ring at the top of the bolt. A quick check would be to loosen the center bolt, lower the canister and see if it's full. If that all looks good, another thing I would try is to remove the fuel inlet from the fuel tank. They have been known to have small rust pin holes near the top of the pick-up tube. If you don't find anything questionable there, I would remove the suction line from the fuel filter, plumb in a line to a small fuel can and see how it runs. One last thing I would check would be the o-rings on the fuel primer. I don't think these would leak enough to cause a "wide open" or nothing situation but they can leak and cause problems.
Check a few things and narrow down the symptoms. The PT pump on these trucks is pretty reliable so let's rule out a few things before you start going down that road.
