If the pedal doesnt go rock hard after a few pushes with the engine off then there is something up.

The way you test the servo is to run the engine, switch off, and it should retain the vacuum inside so you have a few presses until it goes hard, and then you push and hold the pedal and re-start and it should sink.

If the pedal goes rock hard as soon as you turn off, or if you hold your foot on it and switch off and the pedal forces your foot up then the servos leaking.

However if the pedal will not go firm then that test can't be done. You need to check for leaking calipers/wheel cylinders, split/corroded pipes, and a failing master cylinder.

A pressure bleeder is sometimes helpful.....