96 F-150
125K miles
when brakes applied, a mechanical creaking sound is heard.
I can feel it through the pedal.
Brake performance not noticeably degraded.
about 10 creaking clicks not unlike a rusty door being opened slowly
repeats about every ten seconds with brakes applied

not as noticeable when brakes released.

I'm thinking maybe a master cylinder.
Any help would be appreciated.

 

The easiest thing to do would be to lube stuff. Start with the push-rod that is attached to the pedal at the pedal with some white lithium grease. The next most common cause would be rusted up caliper sliders...take the pads out, use some 80-grit sandpaper and clean those things up like new, then apply some synthetic brake lube and you should be golden. Of course the rear brakes could do the same thing, there are 6 contact points on the rear drums, 3 under each shoe that you can lift up the shoe a little bit, and put some lube on with a screwdriver.

 

The sound keeps repeating itself automatically with the brake pedal applied.
about every ten seconds.

 

You mean you hold the brake pedal down...and a creaking sound is heard every ten seconds, and that ten seconds has no relation to speed at all? Man...I think you're hearing stuff.

Anyway, sounds like the drums are out of round to me. You do have drums, right? When you're driving in a parking lot, apply your e-brake and listen ... that'll tell you front or rear right then and there...then get back to us.

Your master cylinder won't creak every ten seconds btw.

 

Check your ball joints and radius arm bushings.

 

When I'm at a stoplight, and I apply the brakes, the sound repeats itself four or five times and then stops - as if something is pumping up to pressure.

 

10 seconds is a long time to be hitting the break. id check all suspension parts,calipers pins, caliper bracket, not sure if 96 has the dust shield but sometimes those rust out where it bolts up and start flapping around and making a racket.