1992 olds regency ninty eight. Break pedal seems to go down futher than it should when applying. when stopped at a light brake pedal seems to slip going further down some. Checked for leaks on brake lines didnt find any. Brake fluid is full in master cylinder. not sure what could cause this to happen.

If master cylinder is bad. the pedal will go to floor running or not.when the pedal goes down at a standing stop it is usually the master cylinder thats bad. and losing vacuum. thats if there are no other leaks at wheel cylinders etc. if putting on a new master cylinder, do not forget to bench bleed it! heres how. put the master cylinder in the vise, fill it full of fluid then pump the plunger with a dowel or something soft as to not ruin plunger part.do not let it run dry ! when fluid comes out from master cylinder and no more air is in it. fill it up and put on car. heres what I do! remove the lines from old cylinder on car. as long as you do not lose any fluid doing this, there is no need to bleed air from brakes when done. after on car make sure master cylinder is full of fluid. start the car and pump the brakes a few times, then you should be all set.