Why Does the Brake Pedal Stiffen Up When the Engine is Turned Off?
When the engine is turned off, you may notice the brake pedal stiffen up. This is a normal phenomenon that happens due to the loss of vacuum assistance provided by the brake booster. While modern vehicles usually rely on this assist, the brakes still function, albeit with more effort. Let's dive deeper into how this works and what to expect.
Brake Booster Function
Most modern vehicles utilize a vacuum brake booster to help reduce the effort needed to press the brake pedal. The brake booster is typically connected to the engine's intake manifold, which creates a vacuum when the engine is running.
When the Engine is Off
When the engine is turned off, the vacuum source is no longer available. This means the brake booster cannot provide the same level of assistance, making the brake pedal feel significantly stiffer. Instead of relying on the booster, you are directly activating the master cylinder, which requires more force.
Pneumatic and Hydraulic Systems in Brakes
The braking system in a vehicle is a combination of both a pneumatic and hydraulic system. When the engine is turned on, it powers the pneumatic system, which extracts air from the salvo, thereby reducing the pressure needed to be exerted on the pedal. However, when the engine is turned off, the pneumatic system goes off, and the salvo actuates the hydraulic system, increasing the pressure on the pedal, making it feel stiffer.
Manual Transmission and Brakes
Interestingly, this phenomenon occurs in both manual and automatic transmissions. In a manual transmission, when the engine is off, the vacuum that the engine creates via a hose from the inlet manifold to the brake booster is no longer available. This loss of vacuum makes the brake pedal harder to push. Even though the brakes will still work, the pedal will feel significantly harder.
Journey Through the Brake Booster System
In vehicles with power-assisted brakes, there is no direct connection between the pedal and master cylinder piston. When the brakes are applied, an opening in the booster reveals an area behind the servo. Inside the middle of the servo, there is a large rubber or plastic diaphragm, or bellow, with a pushrod at its end, facing the master cylinder piston.
Engine Vacuum and Brake Pressure
On the same side of the servo is the vacuum from the engine. When the valve at the rear of the servo is opened by pedal use, differential air pressure forces the pushrod up against the rear of the master cylinder piston. As the valve opens more, the pressure difference increases, allowing the pushrod to move more. When the brakes are not used, there is only engine vacuum and no pressure difference.
Engine Off and Brake Performance
When the car is off, there is just a small amount of vacuum left in the engine manifold, which may be enough for a push. Otherwise, you have to stand on the brakes to get the pushrod to make contact. This is because the distance between the servo pushrod and the back of the master cylinder piston is considerable. You need to get the pedal to push on the bellow to ensure the pushrod makes contact with the master cylinder. Some cars, like VW, used a hydraulic pump instead of a servo, allowing full brake use if the pump went bad. This pump usually shared power steering fluid or was run off the power steering pump, so if it failed, the braking system would revert to manual function.
Pump and Braking Functionality
If the servo fails, it's a critical issue that needs to be addressed promptly. If not, your braking days could be over, both in terms of effectiveness and safety.
Conclusion: The stiffening of the brake pedal when the engine is off is a normal behavior due to the loss of vacuum assistance from the brake booster. While this does make the braking process harder, remember that the brakes still function effectively, ensuring your safety on the road.