Yes new o-rings will help. That is not a total indication that is has more or less than a half gallon. Altough you will never truely know how much fuel has been ran through it and whether it was run a 290 or 200 or 160....just impossible to tell when buying used, But the orings could have just rotted if the motor sat for a while or might just have been loose from the box.. either way new o-rings are cheaper and will surely solve your problem. For a temporary fix use some silicon gasket maker on the old orings, it will keep a seal but make them tighter.