Brake pedal is inconsistent..420SEL
 

My son's 1989 420SEL is having an issue. Sometimes the brakes work normal then once in a while the pedal needs to go to the floor to stop the car. At first I thought maybe brake booster. But if the booster was bad, wouldn't the problem be consistent? Or could this be a vacuum issue?

Thanks, Mark

Brake pedal
 

You should also check the front wheel bearings for wear and adjustment. If they are very loose it will take a lot of pedal travel to bring the disc rotors back into position.

Try this:

Put very light pressure on the pedal, see if it slowly creeps down.

At any time while it's creeping down, push hard, see if it stops creeping.

This would indicate a typical bad master cylinder.

If it is a booster problem, the amount of force necessary to stop the car would change, not as much the pedal position.