All of the above and then some...
Since it had (supposedly) a brake job done before you bought it, it would STILL behoove you to check it out. Not only are Bronco PO's known for being completely dishonest, but think up new ways to cheap on, and cheat on their buyers as well.
Except for those members here that sell their Broncos that is...
I think we make great PO's myself, but we've seen more horror stories in the last five years than we should see in a whole lifetime.
Even if they did do as advertised, old vehicles are often worked on improperly even by professional mechanics because they don't know the little idiosyncrasies of these old rigs.
A common theme, and consistent with your noise, might be wheel bearings coming loose. Seen it many times, where even the pro mechanic did not have the proper hub/bearing nut socket and used the old "hammer-and-screwdriver" method of adjusting and torquing the nuts. And that ain't never gonna work.
Didn't work in the sixties and seventies, and it's not going to work now unless they substitute a mig welder for the screwdriver and hammer!
So it says to me that you should be checking all of the above.
Yes, it could be a master cylinder. But I don't think I've ever heard of a clunking master.
Could be the brake pedal cracking (well worth finding sooner rather than later!)
Could be the pedal pivot point.
Could be the pedal rod wearing out, or the switch and bushing having worn out or worked loose.
Could be ball joints.
Could be anything in the brakes as others mentioned.
Could be the wheel bearings as stated too.
Could be the coil springs in their buckets (rare on Broncos, but not unheard of)
Could be the radius arm bushings wearing out.
Could be the trackbar bushings (usually don't make noise though)
Could be the trackbar mounts.
Could be body mounts.
Could be a rusty cowl or firewall area letting the panel flex and creak when you hit the brakes.
Could probably be other things not thought of just yet.
One way to tell if it's anything like wheel bearings or other steering/suspension components would be to perform "the test" we talk about a lot here. Where a helper racks the wheel back and forth while you watch all the stuff underneath.
You've probably seen the details described, but if not let us know and we can walk you through the steps.
Good luck.
Paul
