As BigJim says, apply the brake with a firm, steady pressure.
The pedal should go down a bit and STOP. If it slowly keeps sinking, there's a problem.
If there are no external leaks the master is leaking internally. And should be replaced.
If you can pump up the brakes, the rears aren't adjusted properly. Or there's likely still air somewhere. With some systems, and especially ABS, the nominal way to bleed is with a vacuum pump. Hand held ones are probably US$30-40.
If the car has ABS there'll be another module that the master cyl. pipes go to. And it'll have an electrical connection.
