My old cat forklift runs perfectly if the tank is completely full. As the level drops to half it runs out of fuel. I disconnect the tank and find it still has fluid sloshing around inside but it wont come out. I have a second tank that does the same thing. They both have done this for the last five years. Today I tried to open the vent when it was out but half full and there was no (0) pressure. There was no indication of frost on any components or the tank. Suspecting air or water in the tank I turned it upside down and opened the vent again. It started to vent Propane I could tell (smell). I allowed all the propane to vent and the tank turned frosty to the middle of tank. It continued to vent until it ran completely out and the frost began to melt. As the pressure ran out I shut the vent. Is it possible that there was an obstruction in (both) tanks that stopped the fuel from running completely out? Could there be some contamination in the (old) fuel? My fuel comes from several reputable dealers. Both tanks have passed recent inspections. Any suggestions would be apreciated.