Lotsa possibilities here...
Bleed your brakes with the rear of the car in the air, as high as you can get it. Trust me, it helps. Check to see if your caliper pistons are close to being fully extended. You may need a pad shim, depending on how you mounted your caliper brackets. Make sure you have enough fluid volume to make the rear calipers work. Drum brakes with the little old wheel cylinder do not require as much fluid volume as calipers. Last, but not least, check your master cylinder(s)... A residual valve will help if you can get pedal, but then it seems to fade. Good luck!