88 4.3 rear brake problem!! help!

ok i have an 88 4.3 gmc jimmy 4x4. i had to change the rear about a year ago. the problem is, is that the rear brakes dont stop...there is good pressure to the back but the cylinders dont push the pads enough to stop the wheels. i have change(:headbanger ):
Master cylinder - all brake pads on the truck - 2 rear drums - 2 rear cylinders - all clips and springs - the brake booster - distribution block - and all brake lines and alot of brake fluid!!! does any one have any suggestions im driving myself crazy ....i appreciate any 2 cents thanks