Strange Brake Situation

[smogrods]

hey every one ive got a weird one for ya!

My 70 c10 has a rear drum brake that starts dragging after a few hundred feet of driving. and my brake peddle get rock hard. any ideas?

my truck has wilwood single piston imca calipers on the front with 6lug rotor
the rear is factory 2" drum im using a new autozone master form a mid 70s c10 ran through a wilwood prop valve. I bleed the whole system with i replaced the front calipers with my pressure bleeder

any help would be great!

(picture is my truck but i swap master cylinders the wilwood one was no good)