I’m going to assume that this is a pressure nozzle system, though you are using the word “siphon” to pull the fuel through the pump, otherwise if you are running a siphon nozzle system why would you need the Webster Biodiesel pump. Why would you have a valve between the high pressure side of the pump out to nozzle? How are you using the pump? In a single line or return-line setup?
It is a siphon system with a return line. Though I am using a little bit of pressure on the fuel side by restricting the flow of fuel on the return side. It makes a stronger flame. I am using the webster because my suntec started leaking at the shaft seal. I thought it might be that biodiesel properties would be similar to WVO and caused the leak. But your line of questioning leads me to believe that the leak was coincidence/age. So WVO wouldn’t be hard on the seals of a regular pump? I needed a new pump anyway. The Webster is working great, though I still have a small air leak into the system (suction side) and will try to troubleshoot again this weekend.
All oil line connections should be made with flare connectors. All pipe threads need to be sealed with pipe dope like Recto seal or alike. No teflon or teflon based tape or dope. If your still sucking air you can dope the face of the flare fittings with a non-hardening pipe dope like Recto Seal.
One old trick is to squirt engine oil (or water for intake manifolds, but it might be a bad idea here) on each fitting. When the bubbles disappear you probably have hit the one. Be sure that you allow enough time for the bubbles to purge.
Air in the oil lines can cause a lot of trouble, especially when you are feeding your siphon nozzle with pressure. You can find a leak by pressurising the line where you expect to be a leak. However when it’s a very small leak it can be hard to find. Air leaks can also be caused because of a huge vacuum in the supply line. A thin and long line, together with thick cold oil is enough. I had this trouble too. 100 feet of supply line (in one piece!) with the pump directly fitted to the line and still air bubbles… Solved it by using a second pump and motor to supply a 3 gallon can. Two floater switches to control the oil level in the can. Now my siphon gun is drinking out of this can, which is near the burner. When it is very cold the burner needs more oil than the extra pump can deliver. When the burner is off, the second pump runs a few extra minutes to fill the can. Bubbles are relieved in the can. Never had trouble anymore since I divided getting the oil to the burner and supplying it to the nozzle.