What Happens When Power Brakes Go Out

If power brakes go out, the vehicle’s brakes still work, but you’ll need much more pedal force to slow down, stopping distances increase, and the pedal usually feels very hard. Safely press the brake firmly, downshift to use engine braking, apply the parking brake gradually if needed, and pull over to get the system inspected. This article explains what “power brakes” do, how failure feels, why it happens, what to do immediately, and how to prevent it.

What “power brakes” are—and what “going out” actually means

Power brakes use a booster to multiply the force your foot applies to the pedal. Most gasoline cars use a vacuum booster driven by engine vacuum; some trucks use hydroboost powered by the power steering system; newer hybrids and EVs often use an electric brake booster within a brake-by-wire setup. When power assist “goes out,” the booster stops adding force—but the mechanical/hydraulic brakes still function. The result is a much harder pedal and longer stopping distances, not a total loss of braking, unless there is a separate hydraulic failure.

How the car feels when power assist is lost

Drivers typically notice sudden changes in pedal feel and braking performance. Here are the most common signs you’ll experience in the cabin if the booster stops assisting.

• Very hard brake pedal that’s difficult to press.
• Longer stopping distance, especially at low engine vacuum (e.g., while accelerating) or with the engine off.
• Hissing noise near the pedal area (vacuum leak) or a brief assist that quickly fades after one or two presses.
• Warning lights or messages such as “Brake booster,” “Brake system,” “ABS,” or “Stability control” on newer vehicles.
• In hydroboost systems, a heavy steering feel at the same time (shared power steering source).
• In hybrids/EVs, loss of regenerative braking and a sudden change in pedal feel, with the car relying only on friction brakes.

These symptoms indicate loss of assist rather than a complete hydraulic failure. If the pedal sinks to the floor or the vehicle barely slows even with extreme effort, that points to a separate hydraulic issue that requires immediate towing.

Why power brakes can “go out”

Power assist can fail for several reasons, depending on the type of booster your vehicle uses. The list below covers the most frequent causes across systems.

• Vacuum booster issues: split diaphragm, cracked or disconnected vacuum hose, failed one-way check valve, manifold vacuum loss from engine stall, or a failed mechanical/electric vacuum pump (common on turbocharged, diesel, and some modern gas engines).
• Hydroboost problems: broken or slipping accessory belt, low or leaking power steering fluid, failed steering pump, or internal hydroboost unit failure.
• Electric brake booster faults: blown fuse or relay, wiring/connectors, failed booster motor/controller, or 12V system weakness (low battery or charging system fault).
• Aftermarket modifications: intake changes or incorrectly capped ports that reduce available vacuum.
• Contamination or water in brake fluid can exacerbate poor pedal feel and performance, though it’s not a direct cause of assist loss.

Because the assist system is separate from the hydraulic brake plumbing, a fault in the booster usually presents as a hard pedal—not a soft or sinking one.

What to do if power brakes fail while driving

If you suddenly lose assist, you still have brakes—just less help. Follow these steps to stop safely and protect the vehicle and its occupants.

1. Stay calm, keep both hands on the wheel, and press the brake pedal firmly. Use both feet if necessary to increase force.
2. Downshift to use engine braking (select a lower gear in an automatic/manual; use “L” or manual mode if available). This significantly reduces speed without overheating brakes.
3. Apply the parking brake gradually if needed. In modern cars with an electronic parking brake, a long press at speed usually commands a controlled deceleration; with a manual lever, pull slowly and steadily to avoid wheel lock-up.
4. Activate hazard flashers and move to the right lane or shoulder when safe. Use your horn and lights to alert others if traffic is dense.
5. Do not turn the ignition completely off while moving—this can lock steering or disable safety systems. Keep the engine running if possible to preserve any remaining assist.
6. Stop in a safe location, set the parking brake, place the transmission in Park (or in gear for manuals), and call for roadside assistance. Avoid driving further until the cause is diagnosed.

Executing these steps prioritizes a controlled stop and minimizes risk. Once stopped, continuing to drive without assist can be dangerous, especially in traffic or on steep grades.

How it differs by vehicle type

Gasoline vehicles with vacuum boosters

Expect a hard pedal and reduced assist if the engine stalls or there’s a vacuum leak. You may have one or two assisted pedal presses from residual vacuum, then the pedal stiffens considerably.

Diesel and some turbocharged vehicles

These often rely on a dedicated vacuum pump. If that pump fails or a hose/check valve leaks, assist can disappear even with the engine running. Hydroboost-equipped trucks may lose both brake assist and steering assist if a belt or power steering pump fails.

Hybrids and EVs with brake-by-wire

These use electric boosters and blend regenerative with friction braking. If the booster or 12V system falters, the car typically shows warnings and reverts to a fallback hydraulic mode with higher pedal effort and longer stopping distances. Loss of regen also increases stopping distance. Many models include backup power supplies to meet safety standards, but drivers will still notice reduced assist.

At-home checks to confirm assist issues

You can perform a few basic checks to help identify an assist problem before visiting a shop. Use caution and keep hands and tools clear of moving parts.

1. Pedal test: With engine off, pump the brake pedal several times until it’s firm. Hold steady pressure and start the engine. A healthy vacuum booster will let the pedal drop slightly as assist returns. No change suggests a booster or vacuum-supply problem.
2. Listen for hissing at the pedal area (vacuum leak) and inspect the large vacuum hose to the booster and its one-way check valve for cracks, looseness, or disconnection.
3. For hydroboost: Check the power steering fluid level and look for belt damage or leaks around hoses and the pump.
4. For electric boosters: Check related fuses/relays; ensure the 12V battery and charging system are healthy. Low system voltage can disable assist.
5. Scan for diagnostic trouble codes with an OBD-II scanner; modern cars often store codes for booster motors, pressure sensors, or pump performance.

These checks can narrow down the likely cause, but precise diagnosis—especially on brake-by-wire systems—often requires professional tools and procedures.

Repair options and typical costs

Costs vary by vehicle and system type, but these ranges are common in 2024–2025:

The following list outlines typical repair items and ballpark costs you might encounter when fixing power brake issues.

• Vacuum brake booster replacement: roughly $300–$900 parts plus $150–$400 labor.
• Electric brake booster assemblies (common on newer vehicles): about $800–$2,000+ parts, $250–$600 labor.
• Vacuum pump (mechanical or electric): $200–$850 parts, $150–$400 labor.
• Hydroboost unit: $300–$900 parts, $200–$500 labor; power steering pump/hoses $150–$700 parts plus labor.
• Accessory belt/tensioner: $100–$250 parts, $100–$200 labor.
• Brake fluid flush (recommended after major brake work): $100–$160.

Prices depend on brand, model, and parts availability. Complex brake-by-wire modules and calibration can push costs higher on premium or EV platforms.

Prevention and maintenance

Routine care reduces the odds of a sudden loss of assist and keeps braking performance consistent over time.

• Replace brake fluid every 2–3 years (or per the owner’s manual) to maintain system performance and reduce corrosion.
• Inspect vacuum hoses and the booster check valve during regular services; replace any cracked or oil-soaked rubber.
• Maintain proper power steering fluid level and belt condition on hydroboost systems.
• Keep the 12V battery and charging system healthy, especially on hybrids/EVs with electric boosters.
• Avoid coasting with the engine off; it eliminates vacuum assist and may disable power steering and safety systems.
• Address engine vacuum leaks and check-engine lights promptly; reduced vacuum can degrade brake assist.
• Investigate any persistent hissing, hard pedal, or new brake warnings immediately.

Proactive maintenance is cheaper than emergency repairs and helps ensure full assist when you need it most.

Safety and regulatory context

Passenger vehicles sold in the U.S. must retain the ability to brake without power assist; regulations require a manual braking fallback. However, the force needed can be far higher than most drivers expect. Review your owner’s manual for the correct use of the parking brake in emergencies—many electronic systems provide a controlled braking function if you hold the switch. If in doubt, stop driving and arrange a tow.

Summary

When power brakes go out, the braking system still works, but pedal effort rises sharply and stopping distances grow. Expect a hard pedal and possible warnings. Slow safely using firm pedal pressure, downshifting, and, if needed, a gradual parking brake application. Then pull over and have the system inspected. Causes range from vacuum leaks and failed check valves to hydroboost or electric booster faults. Regular inspections, fluid service, healthy belts/hoses, and a good 12V system help prevent assist loss and keep your brakes ready when it matters.

What should you do if the power brakes fail?

If your power brakes fail, stay calm and first attempt to restore power by pumping the brake pedal rapidly. Next, downshift to a lower gear to use engine braking to slow the car. Then, cautiously apply the emergency (or hand) brake, pulling firmly on the lever while pressing the release button to avoid skidding. Finally, use your hazard lights, horn, and steering to steer toward the side of the road or another safe area to stop the vehicle.
 
Steps to Take When Brakes Fail

1. 1. Stay Calm: Panicking can lead to loss of control. 
2. 2. Check for Blockage: Look for debris like a floor mat or other objects wedged under the brake pedal, as this is a common cause of failure. 
3. 3. Pump the Brake Pedal: Push the brake pedal hard and fast to build up hydraulic pressure, which may partially restore braking power. 
4. 4. Downshift: Take your foot off the gas and slowly shift into a lower gear to use engine braking to slow the car. 
5. 5. Apply the Emergency Brake: Pull on the emergency brake lever slowly and steadily, holding the release button to prevent the car from spinning. 
6. 6. Signal for Attention: Turn on your hazard lights and honk the horn to warn other drivers and pedestrians of the emergency. 
7. 7. Steer to Safety: Carefully steer the vehicle toward the right shoulder or an open space to bring it to a stop. 
8. 8. Call for Help: Once stopped, get the vehicle inspected by a professional. 

How do you know if your power brake booster is bad?

Symptoms of a failing power brake booster include a hard-to-press brake pedal, a longer stopping distance, a hissing sound from the engine compartment or under the dashboard, and potential engine stalling or rough idling. You might also see a warning light on the dashboard or notice brake fluid leaks. 
Brake Pedal and Stopping Issues

• Hard Brake Pedal: Opens in new tabThe most obvious symptom is a significantly harder-to-press brake pedal because the booster’s vacuum assist is lost, requiring more force from the driver. 
• Increased Braking Distance: Opens in new tabWithout the booster’s amplification of your foot’s pressure, it takes longer for the vehicle to slow down and stop. 
• High or Inconsistent Pedal Position: Opens in new tabThe brake pedal might sit higher than usual or return slowly to its normal position, indicating a problem with vacuum pressure or the booster’s components. 
• Spongy or Sinking Pedal: Opens in new tabA faulty check valve can lead to a leak, allowing air into the system, which can cause a spongy feel or a sinking pedal when applied. 

Audible and Engine-Related Symptoms

• Hissing Noise: Opens in new tabA distinct hissing or whistling sound, especially when you press the brake pedal, is a strong indicator of a vacuum leak in the booster’s diaphragm. 
• Engine Stalling or Stumbling: Opens in new tabA damaged booster diaphragm can create a significant vacuum leak, disrupting the air-fuel ratio and causing the engine to stumble, misfire, or even stall. 
• Check Engine Light: Opens in new tabIf the vacuum leak is severe enough, it can cause a lean engine condition, triggering the check engine light on your dashboard. 

Other Potential Symptoms 

• Brake Fluid Leaks: Opens in new tabFor hydro-boost (hydraulic) systems, fluid leaks are a possible symptom, while vacuum boosters can also be affected by leaks from other nearby components, like the check valve.
• Failed Power Steering: Opens in new tabHydraulic brake boosters are often linked to the power steering system, and a failure in one can sometimes affect the other.

If you notice any of these symptoms, it is crucial to have your vehicle inspected by a professional mechanic as soon as possible to ensure your safety.

Can you drive without power brakes?

Is it safe to drive without a brake booster? No, it is not safe because in order to stop the car, you’d have to apply a lot more force. To slow down even a little without a power brake booster, you’d have to depress the brake pedal fairly hard.

How much does it cost to fix power brakes?

Average Cost
Depending on the vehicle model and year, replacing the brake power booster can cost between $350 and $1,200.