Fuel Venting from left reserve tank

[GMPoland]

Sorry for the lengthy description. Basically,with the fuel tank selector valves in the off position can fuel be drained in that amount from the gascolators? IF not then where would this fuel come from? The parts and sevice diagrams for this year are not too clear as far as the selector valves are concerned. My thoughts were that the fuel was draining through the bad selector valve into the lines and reserve tank thus overflowing until it either equalized or was empty. The comment about the cracked tank has me more concerned than the possibilty of bad valves. If it is a cracked tank what would be a viable repair or is replacent the only option. thanks