fuel leveling problems

I own a 95 peter 377. we replaced fuel lines from the supply splitter to the engine and both the supply valves and the supply splitter and the return splitter a week ago and left on a load with both tanks full of fuel. truck acted like it was out of fuel and shut down. checked the left tank and it was empty, the right tank was full. had to keep putting fuel in left tank which eventually kept emptying in to the right tank till i got home. checked all fuel lines, checked the the splitters, check the vents and even switched them thinking equalizing problem. i am still having the leveling problem. I checked the the return line and they feel hot, the supply line from the left tank is hot but the supply line was cool to touch, so i turned the valve off on the left tank thus only drawing fuel from the right tank think it was not drawing fuel out of the right tank. the truck seems to be able to run off the right side tank but that does not solve my fuel leveling problems. Any ideas on what else I can do to fix this problem?

 

that is what we thought - the engine runs off the left tank and the right tank being a secondary, but after all day trying to troubleshoot this leveling problem and could not figure why the supply line from the right was cool to the touch while the other 3 lines was hot, so we turned off the supply valve on the left tank while truck was running and left the right supply valve open thinking eventually the truck will shut down because of closing off the left side, well the truck continued running for at least 45 minutes before we called it quits and shut her down until we clear our heads and rethink about this problem.
There are no kinks in fuel lines, no worn out fuel lines (all new), no split fittings, the supply tee is not clogged and the return tee is not clogged, the vents are not clogged, no trash in lines or tanks. This just don't make sense, it has to be a pressure problem, forcing the fuel from left tank into the right tank, right?

 

ok thought about the vent valves earlier today,checked them we even switched them from right to left and the problem was the same, left tank emptying into the right.