1996 Geo Prizm bleeding

First, do not use a Vacula, they really suck, next did you bench bleed the master? Run out of fluid while bleeding? Bleed fronts too? This is a diagonal braking system, so the left rear and right front are paired off the master and right rear and left front are paired, to successfully bleed this, bleed all four wheels AFTER bench bleeding the master, You can gravity bleed it by opening all bleeder screws, keeping the master full, and wait until fluid comes from each bleeder, you can close them as they start to drip. Now bleed by pushing the pedal no more than half a stroke and bleeding at the wheels, RR, LF, LR, RF. See how this works. Also very important for rear drums is the shoe adjustment, this controls pedal height!

Take the master off and re bench bleed it, then follow my instruction WITHOUT running out of fluid again.

Adjustment good on the drums? ABS or not? Pull up on the handbrake, does the pedal improve? If yes the rear brakes are out of adjustment, or the wheel cylinders are leaking(sucking air). If you have brake hose pliers, clAMP OFF ALL FOUR WHEEL BRAKE HOSES, THEN TEST THE PEDAL HEIGHT, IF IT'S very GOOD(NO MOVEMENT) THE MASTER AND LINES ARE AIR FREE, IF NOT KEEP BLEEDING, Now remove the clamps one at a time, while the pedal is held down, when it drops significantly, you have your bad hydraulic unit located. Seals under the caliper dust boots and under the rubber boots on the wheel cyinders need to be checked for fluid dripping.
When you bench bled the master, did you use the finger, hose or brass plug method? I use the brass plugs and gently push the plunger half way in and repeat until the piston no longer moves, this removes all doubt about air trapped in the master.

Have you bled the fronts? How many miles are on these calipers, if over 70,000 I would replace calipers and the flex hoses to be certain. Good pedal with front hoses clamped?