The safety valve does not block fluid its only there to turn the brake light on the dash on to warn the driver. If you think about it when you get a leak in one side it loses pressure and the other side of the M/C that still has pressure pushes the switch valve over. Now if you do not have the safety valve between the two brake systems and you get a leak in one side the other side that normally pushes the switch valve over still has pressure and it will still have pressure wether it pushes a valve over of its a solid line with no valve and not connected to the other side at all. In fact the early systems on some vehichles did not have a switch valve in them and they also had no metering valve because the first one were drum brakes which don't use a metering valve and the prop valve was in the rear line only when it had one on the early systems. Ron