I just finished adding a svo conversion to my 1987 F250 Ford Diesel. It ran fine for about a day. Today after I ran about 50 miles I switched from SVO back to Diesel and the truck quit like it ran out of fuel. I pulled the Diesel fuel line and had fuel. I then pulled the SVO line and had fuel there. Then the truck started fine and ran till I switched over to SVO. To make a long story short after running on Diesel, when I try to run SVO the system creates a vacuum and will not run or start till I open the fuel lines. You can hear the sucking sound when I pull off the SVO line. Then the truck runs fine on Diesel. Does the Injection pump create a vacuum if the pump doesn't supply enough fuel? The SVO pump I'm using is rated at 1.5 to 4 psi. After I start running on Diesel after releaseing the vacuum, the truck runs fine on Diesel. I've converted 2 Isuzu Diesels before but this problem never happened before.