Just wanted to jump in here and check something. First, the drainplug is the tank strainer, so pull that and look for anything obvious. You can just clean it out and reuse it.

Secondly you mentioned you were using SVO. Have you ever run Biodiesel through it? What type of SVO kit are you using, or are you just running it straight with no pre heat or extra filters?

I ask this because BioD is a great solvent and will clean all the gunk that dirty petrodiesel will leave in your tank which can leave the prefilter as well as the tank strainer gummed up although this is rare (the tank strainer) unless you have an algae bloom. SVO is much more likely to cause algae problems than BioD, especially if you are not letting the water settle out before filtering it into your tank. Algae loves SVO!

Also, even though the 123 IP is extremely robust, robust enough to run SVO without a second tank( I have run out of fuel going up the hills of San Francisco and pulled up to the corner market to grab a gallon of Wesson before it died and it got me home just fine. Alot easier than fiddling with the primer pump on the side of the road while waiting for AAA), an additional superfine heated filter and and inline preheater(or a preheater of some sort) are absolutely neccessary. The car will run on cold SVO but it sure won't like it. Even an SVO biodiesel blend is rough on her unless you are living near the equator.

Oh, and the expansion tank is just that. similar to the overflow on your radiator. When the tank is full and it is hot out the expansion tank will fill up. I also had this fail along the seams and fixed it with a little alcohol, sanding and JB weld.

Good luck!

__________________
Jeff
AKA: "El Jefe de Biosmell", at your service.
Locally brewed B100 100% of the time.

83 300TBD-T 300,000+/3+years B100
82 Datsun 720 Longbed 86,000/B100
81 300D AKA: The Turd/parts(gone to the bone yard)
75 O309D AKA:The Blue Turtle/76,000

83 VW Golf Diesel/ Girlfriend's
83 F250/ Girlfriend's
Sold all the gas burners!