Just wanted to give another update for those that helped. My 674 quit on my while discing. Changed fuel filters, got running, quit on me again. This weekend I took the supply line off above the filters and opened the valve under the tank and had really good flow, so I decided to remove the filters. After removing the filters and line to injector pump I blew air through the bleed hole while blocking the other passages (as best as I could). I got a bunch of black slimy stuff out of the top of the primary side (it hung down like black slime). I removed the assembly, washed in gas and blew out with blow off tool. Put her all back together and less than a minute from turning on the fuel I had a 4-inch stream shooting up out of the bleeder hole. After much cranking, cracking lines to let air out and pressuring tank I got her going. Used her for about an hour and a half with no more problems. I guess when I let the fuel get really low while working it sloshed that black stuff out of the tank and into the filter assembly. Thanks to all that helped with advice.

Hi Florida, good to hear you solved the problem.

See following from Wikipedia:

Diesel Fuel: Algae, microbes, and water.
There has been much discussion and misinformation about algae in diesel fuel. Algae require sunlight to live and grow. As there is no sunlight in a closed fuel tank, no algae can survive there. However, some microbes can survive there, and can feed on the diesel fuel.

These microbes form a colony that lives at the fuel/water interface. They grow quite rapidly in warmer temperatures. They can even grow in cold weather when fuel tank heaters are installed. Parts of the colony can break off and clog the fuel lines and fuel filters.

It is possible to either kill this growth with a biocide treatment, or eliminate the water, a necessary component of microbial life. There are a number of biocides on the market, which must be handled very carefully. If a biocide is used, it must be added every time a tank is refilled until the problem is fully resolved.

Biocides attack the cell wall of microbes resulting in lysis, the death of a cell by bursting. The dead cells then gather on the bottom of the fuel tanks and form a sludge, filter clogging will continue after biocide treatment until the sludge has abated.

Given the right conditions microbes will repopulate the tanks and re-treatment with biocides will then be necessary. With repetitive biocide treatments microbes can then form resistance to a particular brand. Trying another brand may resolve this.

JimB