brakes popping Options

[FSX_Celica94GT]

when i last got my brakes redone (brakepads replaced and rotors smoothened [cant remember the rite word for smoothened]) and when i brake a certain amount, something in my back right wheel pops, i think it's the caliper, but i dont kno what would cause that, was it not put on right? my 2 front ones also pop but only when i reverse, and when i get out of my car and release my brake, my front ones pop but only once cuz when i try to do it again it doesnt do it, anyone kno what the problem might be?

[bonzaisushi]

i was told by a toyota mechanic, that when the car goes in reverse and you apply brakes, it spring loads the brakes and adds more force on a certain side.... and vice versa.... when i upgraded my brakes and rotors, i didnt see that and the popping went away....