I don't know if this still applies but I had heard that Morgan used the brake master cylinder from an early Mini that had front disks and rear drum brakes. On the Mini they developed a system with a valve that maintained a light pressure on (if I recall correctly) the drum brake cylinders to stop them retracting completely. This was intended to limit corrosion on the exposed cylinder when the piston retracted. On the Morgan they had apparently reversed the connections so that the rear brakes were actuated by the circuit originally intended for the front brakes on the Mini.
As I said I have no idea if this still applies to the 4 wheeler let alone the 3 wheeler.
The valves your referring to are called residual valves.
They are usually used in applications where the master cylinders are about on the same plane as the wheel cylinders.
As you said, they keep the pressure from bleeding off at the wheel cylinders. This keeps both the disc pads and drum shoes in closer proximity to the rotors/drums so a double pedal pump is not needed.
For disc fronts, they use a 2# residual valve and on rear drums they use a 10# residual valve.
In my street Willys street rod, I had Wilwood 4-piston calipers all around and I had to use 2# valves in both front and rear lines.
