Braking Issues

Bottom hits first?

Cam/bushings worn? That'd make the cam hang lower in the spider.

 

They're the bushings that support the camshaft, inside the tube. One at the slack adjuster end, one at the S end. The rollers are on the ends of the shoes and ride on the S of the cam.

 

I agree with the bushings on the s-cam and brake drum mismatch possibility. Could also be the wrong shoes installed. They make several different styles. Cant always go by vin when getting parts for older trucks cause someone could have changed out housings or wheel ends in the past.
I would think you’d be getting a pretty good vibration in rear when applying brakes if drums or shoes are giving an out-of-round condition.

 

If the threadle valve wasn't releasing pressure I don't think you’d even be able to adjust them cause it would be holding pressure to them all the time and causeing them to drag. Now if the valve was sticking and not allowing the proper psi to flow at the proper time when pressed it will lock up instantly or lag.
I could be thinking it backwards wrong tho.
If top shoes aren't touching and bottoms are its worn out or mismatched parts at the wheel ends.