brakes not acting right??

95 merc sable, 3.0 motor, abs, 4 wheel disc brakes.
sometimes ( becoming more frequent ) when i stop at a light the pedal goes down farther than normal and it sounds like the abs is comming on? the car always stops, but im conserned that one time it might not! but it dosent do it all the time... im thinking maybbe the master cylinder is worn out? possibly causing a pressure loss?? checked the brakes they are about 50%, brake fluid is topped off. any ideas?? also if this car has abs, is it a real chore to bleed the brakes if i replace the master cylinder? thanks in advance!

It sounds like the master cylinder. Bench bleed the cylinder then bleed good at the side fittings. It may work otherwise borrow a scan tool and bleed.

ive heard of using the scan tool to assist brake bleeding, do you know where i can get directions on how to do that?

I have an older OTC scan tool when I purchased the module and cable and smart card it came with instructions. Snap On scanners,OTC and the Ford scanners should have instructions.