Sorry Earthdog your wrong there. Bracers of defense set your BASE armor class to what the bracers are.

If you have a mage with 18 dexterity with no armor, his armor class will be 6 (10 base AC minus the 4 points for dexterity = 6)
If you have a mage with 18 dexterity with bracers AC 8, then his armor class will be 4 (8 base AC from the bracers minus 4 points for dex =4)

Armor usually has no effect on your dexterity bonuses. Dexterity bonuses are figured in AFTER your armor.