Richard raises the issue of shortening the trailing shoe lining to increase brake efficiency, and quotes Roger Moss. You can find the relevant topic by searching Forum for “Single leading shoe brake” a thread started by daveherbet May 18 2009. It’s an interesting discussion and worth looking up.
I believe the idea behind this is that the trailing shoe contributes much less braking effort than the (self-energising) leading shoe. As a result it wears more slowly and eventually causes reduced contact pressure on the leading shoe. Shortening the lining at the trailing edge (cam end) of the trailing shoe causes it wear faster so that the leading shoe retains it’s efficiency longer. I don’t think it will enable the trailing shoe to self-energise – my understanding is the trailing shoe is always being pushed away from the drum. That is why they are relatively ineffective.
Richard’s advice about centring the whole assembly by applying the brake hard before clamping the backplate(s) is mandatory when setting up – also I think whenever the brake is adjusted for lining wear.
The matter of cam design is important and a big subject, brought to my attention by Ken Mercer who used to work in the brake servicing industry. There is a lot of discussion about drum brakes and cams in Graham Blighe’s book “Improving Classic Motorcycles” ( email@example.com ).