A good simple diagnostic test is, with the engine running, apply the brakes firmly and keep adding pressure to the pedal.
Does it stay there firmly ??
Then the problem is not with the hydraulics.
Does it slowly sink toward the floor ??
Then, you either have an internal master-cylinder leak with no noticable decrease in fluid level, or an external line, or wheel-cylinder, leak and be losing fluid from the reservoir.
Does it push back at you, not letting you press it down quickly??
This is hard to explain; but, a sure sign of a ruptured diaphragm in the booster is that, when you try to apply the brakes, it is like the pedal is trying to fight back at you, sort of an intermittent reverse pressure against your foot on the pedal.
To track down bad brakes, start at the rear wheels and work toward the master-cylinder.
Most braking complaints can be fixed with a good adjustment of the brakes.
When the shoes are too far out of adjustment, nothing about the system will work as it should.