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.

Peter

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