I know some of you guys are dieselheads, and frown upon us veggieheads, but I really don't know where else to ask this question. Because of the veggie conversion, all the fuel lines are under vacuum, and the return line is looped while on veggie. I have been having an issue with air building up in the fuel lines, it seems the faster I drive the faster the air becomes a problem. I still haven't ruled out that the switchover valve may be leaking, but I am now wondering if there are any seals in the IP or lift pump area that could leak and cause a slow buildup of air bubbles?

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head

When ever dealing with an alternative fuel system, try for a bench mark to judeg the system ... diesel.
Do the Sureluck Homes method, prove what works and ...
Make a temporary fuel 'tank' (pop bottle) like in the Diesel Giant purge tutorial. If the vehicle works fine using this temp. tank filled with diesel, then you know the IP seals & lines are ok. Then move down the line still using the temp tank and diesel. Your WVO system will never know it now is (temp.) drinking diesel. There is a fuel dye for finding nasty leaks.
Why do you assume your fuel lines are under vacuum? Do you have an aux. electric fuel pump at the tank? With a 'faucet' style aux. pump, the run from the tank to engine compartment is presurized & everything from the IP is presurized.
Without knowing what kind of conversion you have, I guessing, but I bet you have a filter or fitting loose. There are few posts on IP seal problems.
When my OEM prime pump went out, I had air in all the lines. Engine would zoom then steadly loose power, catch again and ...
WVO user to WVO user: "I really like these dino fuel folk! They have chains of little stores across the planet with hot cofee, donuts and bathrooms! But strange short term parking!!"