What engine is on your mower? Sounds like a B&S with the suction type diaphram carb mounted on the tank. Actually, I hate working on small engines, but somehow I seem to have a never ending supply of other peoples mowers that get dropped off for me to "take a look at"...so I guess I'm willing to learn. Now the safety handle...every safety mechanism that I had the "pleasure" of working on, shorted the magneto/coil/electronic ignition when released and some also applied a brake to the flywheel. So I can't see how the engine could continue to run untill it was out of fuel, whether it was gravity fed or suction type. I've found quite a few cap vents blocked, but never did I find it hard to remove the cap due to vacuum in the tank.