Your vehicle has power-assisted brakes that adjust automatically through normal usage.
If the engine is not running or is turned off while driving, the power assist for the brakes will not work. You can still stop your vehicle by applying greater force to the brake pedal than typical. The stopping distance, however, will be longer than with power brakes.
When the engine is not running, the reserve brake power is partially depleted each time the brake pedal is applied. Do not pump the brake pedal when the power assist has been interrupted.
Pump the brake pedal only when necessary to maintain steering control on slippery surfaces.
WARNING
Take the following precautions:
Hyundai Elantra AD (2016-2020) Service Manual: Rear Oil Seal Components and Components Location
Components
1. Rear oil seal
...
Hyundai Elantra AD (2016-2020) Owners Manual: Engine Start/Stop Button
Whenever the front door is opened, the Engine Start/Stop button will illuminate
and will go off 30 seconds after the door is closed.
WARNING
To reduce risk of serious injury or death, NEVER allow children or any person
who is unfamiliar with the vehicle to touch the Engine Start/Stop button ...