Thought perhaps that I would add/update this thread with my findings so that anyone finding this thread can read further:
Well yes, as stated the leading shoes on the rear brakes are indeed fitted with thicker brake lining materiel, more than twice as thick in fact from new. I have never been able to obtain a definitive answer as to what minimum thickness due to wear is the lowest limit thickness of lining material. However the leading shoes do tend to have the greater wear and I have reached the conclusion that the trailing shoes must have a harder material for their lining as having a duel piston which if working correctly will apply equal pressure to both shoes as they engage with the drums. so, I gauge my wear limit to be when the lead shoe approaches the thickness of the trailing shoe, about 3mm remaining. The lead shoe is also the most effective shoe for breaking efficiency or in the case of drums fitted with two separate pistons the one leading edge of each shoe. I feel that this is part of the design of the rear drums in that the breaking effort is almost in total acquired by the leading shoe with the trailing shoe only applying the required force to allow this to take place and the harder lining material which not only restricts wear but also reduces harsh breaking to the light rear end, this prevents the rear of the car breaking away which is more likely to happen when pushing your driving style.
Disc brake conversions.... yes they work and yes you need to fit an adjustable regulator to restrict over breaking at the rear.
Disc brake conversions work best with the Quaife axle the BTM has issues in that it has a built in end float which should be set at 1.2mm (not adjustable) which is not too bad although I hate not being able to adjust this out. However it is not unusual for this end float to exceed 6mm now this is not noticeable under breaking with drums fitted but with discs you have to centralise the disc and calliper alignment and racers using them have to dab the breaks quickly before applying pressure to centralise things first, not so easy on cornering though! The drum set up is perfectly alright if serviced correctly. The pistons do seize up and in my experience its not due to rust, rather it's like they gum up as the piston lubricant becomes sticky and they can be well stuck. Dealers seem to have two options for fixing them, firstly they just change the cylinders with the associated brake rebuild and a lovely remuneration for them into the bargain as well! and second option is they pop the drums off and tap the pistons from side to side until its loose enough to pass a brake test and that won't last for long until they re-seize again! there are a lot of later classics out there without the rear brakes working properly as the front disc's are good enough to stop a light car like the MOG. A hand brake test makes you believe they are working fine but they certainly are not!
Is there a solution which won't break the bank? Well yes there is, I have never stripped a modern classic whereby the rear cylinders have required anything more than a service and a repair kit which is readily available for under £30.00. Yes the overly complicated rear drums are a pain in the proverbial but learn how to do this job and things start to look a lot better and cheaper too. The auto adjuster is also weak and requires to be cleaned and serviced so this can be done at the same time. I keep a set of spare cylinders serviced and ready to go but in reality a simple clean and polish with the cylinder body still attached to the backplate is all that's generally needed. Just one further note. This gumming up of the cylinders does not seem to be as a result of hydroscopic water absorption of the brake fluid causing rusting. I have for many years used red rubber lubricant and I'm sure this sticky gloop is responsible. The rear brakes are so lightly used the heat never really builds up in the rear drums and its not combined into the brake fluid as you would expect so I am now trying alternatives but I remember that as a youngster servicing older generation cars at work 1960-70's we would use vaseline to lubricate the cylinders and never had any trouble and these cars would have been much like a Morgan in the level of rear braking effort and believe me we seldom fitted new cylinders and would fettle and fit repair kits unless they were totally shot.
