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Most of the braking in a car (or a bike) is performed by the front wheels. This is because under brakes there is a major transfer of vehicle weight to the front. This is why the front of a vehicle dips and the rear rises. If you stamp on the brakes, as in panic, there is a tendency for the front wheels to lock up because the weight of the vehicle has not had time to transfer and create the downward forces on the tyres for effective braking. Under correct controlled braking the brakes are applied progressively with increasing force as weight transfer increases the grip of the front tyres.

Because of the reduced weight on the rear tyres as the back of the car tends to lift you require less braking forces at the rear or you would lock up the rear wheels. Manufacturers go to a great deal of effort to get this balance between front and rear wheels correct. Usually smaller disks and callipers are used in all disk systems or less powerful rear brakes in all drum brake systems. Many light vehicles use disks at the front and the less powerful drum brakes at the rear. Drums also require lower hydraulic pressure and have the advantage of being very easy to use for handbrake systems.

Race cars often have a driver controlled proportioning valve to effectively limit the rear braking forces.

To return to the Morgan: It has been said that for road use rear discs would be so lightly used that if a car was only used on relatively short trips there may not be enough brake use to completely clear corrosion from the rear disks. The end result could be the build up of corroded patches on the rear disks that would eventually lead to uneven braking.

With a light weight car such as the Morgan rear drums are perfectly adequate for road use and cost less that rear disks (which would also need a separate handbrake system). For road use it is difficult to immagine the circumstances under which you would ever get any fade from the rear brakes. If you doubt me try touching the rear drums after a period of driving on a mountain road. I have and I can assure you that they do not get hot enough to cause fade.

Improved front brakes would probably be the first thing I would consider. Unless you constantly drive in a spirited manner or intend to get involved in competition, perhaps the rear discs on the AR P4 are more of a fashion statement than a necessity.

Just my idea

I had the predecessor AD07 on a Elise 8 years ago, a very good tyre. Test reviews say that the AD08 is even better, with a really good grip, not quite as much as an AD04 (famous semis) but AD08 is very good in the wet as well, where an AD04 is almost hard to control. AD08 is the best road performer by Yokohama and could be also used occasionally on track days.
From memory the first Aero-Plus8 were OEM fitted with this tyre as well.

am really enjoying the high resolution pics

also very heartened by the quality of the construction and pleased to see braided brake lines

in pic 4 (front bulkhead), you can see the red paint had come off at the bottom edge of the bulkhead .. and the metal looks rusty underneath; worrying!

the oil tank will have to go to the left on RHD cars as otherwise no space for steering column and pedals

delighted I have a slot

as it's marketed as a track focussed car with strong road going capability it needed to have good rear brakes

You can relax.....that is mastic sealant. The bulkhead is alloy by the way.

looking at the pics, the oil tank for LHD cars is right next to the exhaust manifolds ..