93 explorer brake problem

my 93 explorer has recently developed an annoying problem. the brakes when cold work well with no noise or vibration when stopping but after about 20 minutes of driving and braking ,when the vehicle is almost stopped there will be what is best described as a loud dull chirp and at the same time the pedal will sink a bit. it sounds that it is coming from the front left. i pulled off the wheel , inspected the pads[ they were fine] and lubricated the sliders with no change. i am leaning toward a caliper or possibly the a.b.s. so any advice or ideas or much appreciated as i do not wish to just keep replacing until i find the problem thanks sam

Try disabling the ABS system by pulling the fuse. The system will operate normally without ABS (other than no ABS function) if the sound disappears you have isolated your problem.