I would suggest rechecking the assy since if it wont even hold with the parking brake something is wrong & it ain't the shoe composition. Make sure the 6 bare smooth "pads" on each backing plate do not have deep grooves in them which can hang up a shoe. I'd say the shoes ain't tight enough. I put the rear up on stands & idling in gear with a helper in the dr seat you adj each side till you reach a point where you can hear a slight "tick" at 1 point in the tires' revolution. He hits the brake/stops everything you spoon it up several clicks then he releases & you listen for the "tick" then he hits the brake & repeat. Good jack stands/safety first. If your brake shop arc's shoes to the respective drum that's a good idea, here it's $5 a corner which ain't nothing. Holler how it goes. EDIT Aside from the parking brake issue how is the normal braking? & how much pedal reserve? (distance from pedal to floor when depressed)