[Eman]

I'm confused about the linelock issue and why you would run the rears loose.
Drum brakes are self energizing, the frt shoe contacts the drum and as it is forced to rotate it applies increased pressure to the rear shoe. This is why they work well to stage a car.
On my rear drums I use 2 secondary shoes on each side, you buy 2 sets of shoes so you can only use the long lining ones. Go to a real parts store and try and cross reference wheel cylinders. The larger the bore wheel cylinder the more pressure will be applied. Basic hydraulics, the surface area of the wheel cyl piston multiplies the force.