Brake Pedal Moaning

I found another thread, but I'm not quite sure the symptoms are the same, so I'm starting a new question...

2001 Rodeo LS, V6. 53,000 miles.

From time to time, the brake pedal moans, creaks, make a low pitched noise. It sounds as if the noise is coming from the foot well inside the truck, not from the engine compartment. It sounds to me like rubber on metal or something along those lines. I hope that makes sense... Is there a bushing somewhere on the pedal that could be rubbing, or am I most likely looking at a master cylinder problem like another thread suggested?

I have not had any service done to the brakes yet, and I'm the original owner. The pads still look fine to me, but the fluid is original, and likely overdue for a change.

Thoughts?

Anyone have any ideas please?

Check all your vacuum lines, especially the big one going to the master cylinder. It's not a fluid or pad issue. I had an old Celebrity (Chevy) that did the same sort of thing, and it seemed to be a vacuum problem although I never really got around to fixing it.
Tom C.