I just finished my conversion on my 83 300D turbo diesel benz. I am using a looped return with 2 Hydraforce 3-port solenoid valves. For the past week, I keep checking my oil tank and it is still full. Although, when running on oil, I see through my site tube it is clearly running brown oil and the exhaust smells like oil. Today I did a test and only flipped the return switch to loop the return while running on diesel and I saw the oil tank rise and overflow. For some reason, when I am looping my return, it is not looping but returning diesel to my oil tank. The reason I was running with only the loop on after I switch back to diesel was to try and purge any remaining oil out of the lines and loop it back to the engine...but it appears to be going right back to the oil tank. This phenomenon does not happen when running on oil, it seems to loop through back to engine. How can I prevent this from happening? It seems to be creating reverse pressure/flow. I have the T right after my Vegmax oil filter. Is this a common issue? Any help is appreciated.

Depending on how you have your T's set up, this is what is going to happen when you are on on looped return and are feeding from diesel. I'm assuming that the place where the return T's back into the input side of the loop is before the input selctor valve. If this is so, when you have that valve selected to diesel, the fuel going into the looped return can't actually get used since that side of the valve is closed. So the fuel has to go somewhere, and goes to your tank. If you put that T in after the valve, then you can loop the return on either fuel. Try sketching the exact fuel flow of your system in all four modes.

Good Luck.