power booster question

[frankyjoet]

I changed the rear brakes to disc, i now have 4 disc wheels. my 73 gmc truck brakes need to be bled. When i tried to bleed them i hear an air noise from the power booster rod. now since i replaced all brake rotors and calipers, i assume or think that there is air in the brake lines. the first set you do is the rear, then alternate between the front, is this correct? i read from a good source that if the brake pedal is HARD, then the booster is bad. Also a little noise is normal. My ? is the booster ok? If so i can bleed alll the brakes. Thanks for any help.