My god, have you people never pulled a master cylinder apart? You can show all the pics you want but they dont even show clearly what's going on. THERE IS BRAKE FLIUD BETWEEN THE FRONT AND BACK PISTONS IN THE MASTER CYLINDER. The very reason you "bench" bleed before installation. If you lose that hydraulic lock for whatever reason there's nothing to push the foward piston until the contact by the rearward piston (no mechanical conection with the brake rod till it bottoms out) which will give marginal braking at LOW speed. Without the safety valve, fluid and pressure are lost every time you pump the pedal. For the ones that don't believe this fine, do what you want, it's your life. The newbe, put the safety valve in the system, to hec with the light.
Something about a water trougth and a horse.