Originally Posted by cerial
Semi brakes use the same size shoes in all there drums.
Most wheel cylinders are double acting.
I have no experience with semi truck brakes. I DO know that GM A-body brake shoes are not anchored to the backing plate at the bottom. The upper edge of the front shoe is pushed outward by the wheel cylinder. Rotation of the drum rotates the two shoes and self adjuster towards the rear pin on the wheel cylinder.
This rotation is how the self adjusters work.
Yes, the wheel cylinder is double-acting, but the pressure inside is the same on both pins, and the added friction force rotating the shoe assembly around pushes back against that rear pin.
I'll ask again: since GM pinches every penny possible, why would they go to the expense of having to stock two different shoes for primary and secondary if a single common shoe would work?
To the OP, since I missed this in your first post originally, I don't know what you're reading, but the shoe with the SMALLER lining is the PRIMARY shoe and goes to the front.