This old tank that I brought home still has some propane left in it but I don't know how much. I rolled it out of my friends truck and into mine, got home, backed into the yard and pushed onto the grass out of the back of the truck. I got it in the back yard, wanting to remove the valvesn to fill with soap and water to let sit for awhile. Is it safe to vent the tank from propane? I think it's residual because I moved the tank around to shake it and didnt hear anything inside.