Yep Spot on icepack, as the adjuster is allowed to take up a new position so does the cylinder pistons. so more brake fluid from the reservoir remains present in the cylinder body but unlike the front calliper pistons the extra fluid required to hold the pistons in the rear drums in an Extended/ing position is negligible in comparison to the front ( hence brake fluid level drop).
This effects the position of the shoes within the drums as they wear but still keeps the contact of the friction material fully in contact with the drum surface. The metal part of the shoe which holds the friction material is circumferentially similar in diameter to the mean internal diameter of the hub, so initial wear to new brake shoes tends to be to the outer ends of the shoes and this then equals out as the shoe wears in. Eventually as the shoe reaches its lowest permissible thickness it will be very close to leaving an equal 1-2mm around the whole shoe. These wear patterns allow you to make an informed diagnosis of what might be wrong with your rear drum brakes. For example a worn shoe showing equal amounts of friction material left at both outer ends of a shoe yet worn so there is hardly any material left in the central area would indicate that your drums have gone beyond their maximium wear diameter, ie, the internal drum diameter has worn and now its internal size is to large. Of course there are several indications on this theme and it's probably worth looking them up as so many times you see rear drums having had work done to them yet clearly the initial problem was never addressed. Yes I suppose it is a bit of a dark art but probably self adjusters didn't help. When shoes had to be manually adjusted to bring the friction material into the desired position, more of us were aware of our brakes performance and to a greater extent what was probably needed to sort them out. It was not unusual for your dad to get you out on a Saturday afternoon when he got back from work to just watch him show you how to adjust the brakes. Nor was it unusual to end up stripping the whole thing down to service a piston........ But that's for another day!
