Master Cylinder or Brake Booster

Well, why did you change pads/shoes? Did the problem start after the repair?

A bad master cylinder doesn't make a sucking sound, that's a failed booster diaphragm, I'd guess.

I had a Blazer that did this. A new booster fixed it.

Good luck.

If your brake booster is, like most, actuated by engine vacuum, then a leak in the diaphragm could indeed cause a sucking sound.

A secondary symptom which you might want to check for is that if you apply the brakes shortly after stopping the engine, you should feel a rapid increase in the pedal resistance (back force) as the vacuum supply bleeds down through the leak in the diaphragm and you lose power assist.

It is probably a bad booster but it would be better if you would have giving a little more info like yr. make and model you would receive a lot more info that could be a lot more accurate for your specific vehicle.

Does the pedal drop out to the floor, or is it really stiff?

The "air" noise can be a symptom of a leaking booster.

More information on both the vehicle and the symptoms, please.

Its a bad booster. Watch out, the diaphram will usually fail completely as it gets worse. Then you'll only have manual brakes. If they're not perfect, you may have Very little brakes left at that point.

check if your brake fluid has fallen way below the refill line , if it has then you are sucking air into your braking system. and getting about nil breaking effect

Solution top up the fluid ,pump the brake pedal until it is relatively stiff.

Normally a badly leaking brake booster will make the engine run rough. You can always unplug the vacuum hose and put a bolt in it for test purposes.

Sounds like what happens when air contaminates the brake fluid upon replacing B. shoes or B.pads, as the wheel cylinders expand their pistons and they retract when the shoes or pads are installed, air is introduced through that contraction process. thereby resulting in a spongy B.pedal and low pressure applied to the process . Just one possibility.