brakes/pedal

My piece of junk corolla is having an issue you push on the brake and when you get to "almost all the way down" it makes a suction like sound. The pedal goes down really far. There is fluid in the resevoir. If there was a leak couldn't I see bubbles as it sucks air into the lines through the master cylinder and resevoir? I don't remember how to tell if the master cylinder was bad but I thought it had to do with when the car was off and pushing the pedal over and over to build pressure, which it does while its off. They may not have anything to do with testing the master cylinder. haha.

Does your car have power brakes ? That " suction sound " might indicate a bad brake booster or a leak in the hose providing vacuum to the booster ? Since the sound began , have the brakes been less effective and require a lot of pressure , from your foot , to stop your car ? That would indicate the booster isn't doing it's job and possibly show the booster ( or associated part ) to be bad .