rear brakes

i have a 93 buick skylark G.S.i've put all new brakes and rotors and brake drums. but every once in awhile when you hit brakes theyll kinda lock up. dose anyone out there had this problem? i'm thinking ppv but still not sure i would appreciate any help?

very common issue on these cars. The brake drums don't self clean very well, and brake dust builds up causing the brakes to get grabby.