A brake pedal that feels rock-hard usually means your power brake assist is gone. The car will still stop, but it takes a lot more leg force. Here are the most likely causes ranked by how often they turn out to be the problem.
The vacuum booster amplifies pedal force. A torn diaphragm or stuck valve kills boost, leaving a hard pedal that needs heavy effort.
The hose between the intake and the booster cracks or pops off. Easy fix if you can spot the leak with engine running.
A one-way valve holds vacuum in the booster when the engine is off. If it leaks, the booster bleeds down and the pedal stiffens.
Diesels and many turbo gas engines use an electric or mechanical vacuum pump. When it fails, the booster cannot pull a vacuum.
A caliper stuck partly applied makes the pedal feel hard and high because the slack is gone. Often paired with one hot wheel.
You have to stand on the pedal to slow the car, the brake warning or ABS light is on, or the pedal pulses oddly. Reduced brake force can extend stopping distance dramatically. Drive only to a shop and slowly.
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Your power assist is gone. The hydraulic system still works, but the booster is not multiplying your foot force. Replace the booster or fix the vacuum supply.
Parts run $150-$450 and labor is $200-$500. Total typically $400-$900 including a fluid bleed.
Yes. The booster needs strong, steady vacuum. A cracked or disconnected hose between the intake and booster will make the pedal hard.
Only short distances at low speed to get to a shop. Your stopping distance can be 2-3x normal, which is a serious risk on the highway.
Pump the pedal 5 times with the engine off, then hold pedal pressure and start the engine. The pedal should sink slightly. If it stays hard, the booster is bad.