Note for North American readers: some South African and Australian Valiants, such as the one brewil is talking about, had a remote brake booster setup available as factory or dealer-installed equipment, and similar setups were (and maybe still are) available for aftermarket installation. The booster mounts on the inner fender. The master cylinder is mounted directly to the firewall, operated by a pushrod pushed by the brake pedal. The hydraulic line that would ordinarily serve the front (disc) brakes goes instead to a slave cylinder on the input side of the remote booster. The booster operates another master cylinder on its output side, which serves the front disc brakes. In this way, you get boosted front disc brakes and unboosted rear drums. Good setup for avoiding premature rear lockup.
Brewil: I think you will likely want to either keep the front boost as per existing configuration, or change to 4-wheel boost. It's all down to how much pedal effort and travel you want.