Crown does in fact also make the two REAR brake lines that go to each wheel from the proportioning block above the rear axle. That being said, those two new lines can be used ONLY by cutting the oem brake line above the rear axle back a few inches back from end and then re flaring the oem brake line to accept the Crown brake lines. In the process of cutting and reflaring the steel brake line, you have to carefully remove the epoxy coating from it and then also reapply some sort of rust preventative afterwards. In other words, due to the design of the oem brake line, replacing those two rear lines CAN be done, but it is simply more work so a lot of people choose NOT to go that route.
As others have noted, Crown customer support is EXCELLENT and if you call them, they can explain the situation more clearly then I have.