juice/mechanical brakes?

[48pan]

I just found a box of parts from when I moved in 1980. In there was a brand new rear brake master cylinder. Does anyone remember when they changed from mechanical to juice brakes with the rear brake?

[Dogbone45ACP]

1958 for big twins. The drum brake cylinder has a residual valve in to keep a little pressure on the wheel cylinder. The disc brake cyl. does not have it.