Why is the brake hard when starting the car overnight?
2 Answers
The reasons for the hard brake when starting the car overnight include a lack of vacuum in the vacuum booster pump or air leakage/damage in the vacuum booster pump. Lack of vacuum: When there is no vacuum in the vacuum booster pump of the brake system, the vacuum booster pump cannot function properly, resulting in the brake pedal being hard to press. Air leakage or damage in the vacuum booster pump: If the sealing ring of the vacuum booster pump is worn or lacks elasticity, it should be replaced promptly. If there is air leakage in the pipeline of the vacuum booster pump, the leaking pipeline must be replaced. If the interface of the vacuum booster pump is loose or damaged, it needs to be addressed accordingly.
I reckon this issue is related to the vacuum booster pump in the brake system. When the engine starts, it relies on the negative pressure in the intake manifold to reduce braking effort. If you can't press the brake pedal in the morning, it's most likely a problem with that vacuum hose—maybe it's aged and developed a small crack, the hose connector is loose, or the one-way valve inside that maintains vacuum has failed. Just like a punctured balloon can't hold air, the vacuum pressure leaks out overnight. My old car had this exact problem last winter, and replacing the sealing ring in the line fixed it right up. If you're driving a turbocharged car, you should additionally check if the vacuum pump is working properly to maintain pressure—this component is most prone to failure during cold starts.
