First check is your servo,
start the engine and rev it a little than switch off.
Now press the brake pedal several times then hold it hard down with your foot,
now while still pressing the pedal turn the key and start it up.
the pedal "should" dip a little lower under your foot.
If it doesn't then check the vacuum from the pump to the brake servo,
With the engine running and the big pipe removed from the brake servo if you cover the end with you thumb you should feel the pipe sucking.
If so then it's either the servo at fault or the brake master cylinder.