Alt. guage discharging all the time???

ok pulled the alternator and took it down to advance auto, tested out fine on the machine, picked up a voltage reg. while i was there since it was cheap enough and mine looked pretty rough. Came home and put the alternator back on along with the new volt reg, went to fire it up and nothing but clicking. Tested the battery with the terminals off of it and it had 12.4ish volts. Kept randomly checking it over a period of about 30 min. and it was steadily discharging with no terminals hooked up. Took it up to the store and it tested bad so i replaced it. I get the new one back home, hook everything up, turn the key to the on position and same thing, moves to the left of center discharging. Car did fire right up but everytime i either depressed the brake pedal, turned lights on, etc it would discharge almost all the way to the left. Could the alternator be bad even though it was checked? it looks old, part number 7509, a reman from advance from previous owner.....