Bill B,

Any car that has drum brakes on any wheels requires a residual pressure valve in the master cylinder port that supplies the drum brakes.
If your car has a split system with discs on the front and drums on the rear you still require the valve in the master cylinder port supplying the drum brakes.
You don't need the valve in the disc brake pipe as the seals work in quite a different way to drum brake wheel cylinders which require pressure to keep the cup seals expanded.
This is the purpose of the residual pressure valve which keeps a low pressure in the brake line to keep the seals expanded slightly but not enough to apply the brakes but stop air entering when the wheel cylinder pistons move back and forth slightly due to brake drums not being perfectly round.
On most Aus minis this valve is internal to the master cylinder. (The first part that goes inside the master cylinder before the spring, piston seals etc)
You don't need one in the clutch system as the slave cylinder isn't moving unless the clutch pedal is pressed.
On cars with split systems, ie cars with 2 internal pistons and separate brake pipes from the master cylinder, the valve is usually in the outlet to the rear brake pipework inside the pipe union.
My old Nissan Skyline, circa 1979, had a rubber valve inside the outlet union to the rear brakes. Similarly in a 1970 Valiant I had.

RonR

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68 build Mk1 Cooper S
72 Clubman GT project. Total restoration required