are you sure the primary and secondary shoes are installed in the correct place, as well as the correct return springs on the shoes? the long shoe lining usually goes on the rearward side. the weaker spring usually goes on the frontward side. also check the backing plates to ensure there are not worn out grooves where the shoes sit against the backing plate. this is common and the shoes "fall into" the worn grooves and then when they come out, with a little pedal application, they can grab the drum and cause erratic brake action. a smooth backing plate and some brake lubricant applied in that spot is normal, not lube on the linings, just the steel backing part of the shoe. it usually has a small expanded area where it rubs on the backing plate. below is a link to the original manual and it explains self energizing brakes better than I could.basically the front shoe has a weaker spring so it tends to push out first, grabbing the drum and slightly turning the shoes on the backing plate so they jam against the mounting pin at the top of the backing plate. that's the reason for more lining on the rear shoe as well. maybe just read the link, lol
how far does the pedal travel before you get some feedback? you may want to check all the linkages and return springs right from the pedals down. ensure all the pivot points etc are all free and easy moving, no binding etc. ensure the pedal returns fully but also ensure the master cylinder returns fully. all it takes is a little slop in a linkage point and a sticky pivot so the master cyl doesn't return
http://chevy.oldcarmanualproject.com...4ctsm0502.html