The round springs/cups keep the shoe held against the backing plate, but the rotor also keeps them from going too far. From the sound of it, the p-brake is not adjusted tight enough, and the lever is popping over the edge of the shoe. Spin the adjuster until the rotor locks down, then back it off until you can just turn the rotor. Same as adjusting "normal" rear drum brakes. This adjustment is best done with the halfshaft(s) disconnected, so there's no drag/binding from them. Or...jack the t-arm on that side up and get the weight off of the halfshaft.
Once you have both p-brakes adjusted properly, then adjust the center cable from the p-brake handle. It should lock the rear brakes within 8-10 clicks. Sooner is better, tho. If you have to adjust the center cable all the way up to the end of the threads, there is a problem somewhere.
The linkage to engage the e-brakes on these cars is cheesy as heck to begin with....but it was cheap to make. When adjusted properly, the p-brake will hold solidly in reverse...forward, not so much.
Don't plan on being able to lock the rear brakes up with the p-brake at speed....it ain't gunna happen. The p-brake is only good for parking....might help a little if your regular brakes fade, or fail, but not much.
______________
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"