So with the line lock blocking that front brake line, and the line being already filled with fluid, there would be zero piston movement for the front brakes, yet the pedal moved enough to apply the back brakes.

That is because the flow of those two lines is connected internally, and they have to be. If there was no internal flow/connection, the pedal wouldn't move enough to apply the back brakes. This also means if there was a leak in that front line, or the rear line, the pedal would go all the way to the floor.

As state above, the switch valve shuttles to the leaking (low pressure) side to cut off flow down stream from the valve, allowing all master flow to go to the non leaking side.

This topic comes up from time to time, I'm just trying to inform folks they ain't as safe as they think they are. But, no one will open a bleeder to test, so this post will end like all the others.

Stay safe, hope for better days ahead, and a spring with a racing season to it..

Edit: It just occurred to me that most all of us have already tested this thing as I wanted to. You change out your rear, or something on the front end and you gotta bleed the brakes. So, with a guy on the bleeder, you push on the pedal and it goes to the floor, eh? You floor it a couple times, refill the master and you got it. BUT, the pedal went to the floor! Would it do that IF you had redundancy?

[so was it Billy Mays that said this??]---BUT WAIT, THERES MORE! Even if you have a shuttle valve the pedal will go to the floor when bleeding, what's up with that? That's because when you are bleeding you aren't pushing hard enough to overcome the centering springs on the shuttle valve, SOO, you only have redundancy when you press on the brakes like you are stopping the car.

OK I'm done, wish I could take my car out and my sweetie to the restaurant. Stay safe.