If you read the rest of my post you would see it says all later valves had the prop valve and metering valve in them but the early ones did not. And that pistons does not block fluid from the other side. It is two independent systems. Look at the inside of a dual M/C and you can see they are independent of each other. Yea you will lose half the pedal if one side springs a leak but if you look at the inside of the M/C you can see M/C piston will compress the spring in it on one side and still have the other side with a much lower pedal. I did open a brake line on my car and I still have half my brakes. In fact I had both of my rear bleeders open and pumped my pedal and still have just under a half pedal and front brakes that stopped the car but not as good as all four of course. Ron