A brake pedal that sits high and feels rock-hard - taking a lot of foot force to stop - is the opposite of a soft pedal, and it points to the power assist. Here is why your brake pedal is high and hard.
No booster assist
The power brake booster multiplies your foot force using engine vacuum. If it is not getting vacuum, you get a hard, high pedal and weak braking; see booster vacuum and hard pedal. This is the most common cause.
Where the vacuum is lost
Suspect a failed booster, a bad check valve (that holds vacuum), a cracked or disconnected vacuum hose, or - on some engines - a failing vacuum pump. With the engine off the pedal is always hard; if it stays hard running, the assist is gone.
Test it
Pump the pedal several times (engine off) to bleed vacuum, hold it down, and start the engine - a good system lets the pedal sink as assist returns. No change means a booster or vacuum fault, not the hydraulics.
What to do
Trace the vacuum - hose, check valve, then booster - and repair the source. Braking is much harder and longer without assist, so fix it promptly. Keep the friction brakes fresh with quality pads and hardware.