What happens after the friction between the rotor and brake pad stops the vehicle?

When the friction between the rotor and brake pad comes to a stop, the hydraulic pressure within the braking system decreases. This is because the hydraulic system is designed to apply force through brake fluid, and once the brake pedal is released or when the brakes are no longer engaged, the pressure will naturally drop as it returns to its resting state. The hydraulic pressure is what allows the brake pads to grip the rotors tightly, and when the grip is released, the pressure dissipates.

This drop in hydraulic pressure signaling the cessation of braking allows the vehicle to roll freely once the brakes are no longer actively exerting force on the rotors. It's a critical aspect of how modern braking systems operate, relying on hydraulic principles to ensure that vehicle stopping and starting can occur smoothly and effectively.

The other options represent different scenarios that do not occur as a direct result of the friction stopping. For instance, a locked wheel indicates a loss of control due to excessive braking, which is not the case after friction ceases. The engine stalling is unrelated to braking dynamics and refers more to the engine's operational state, while the caliper warming up can occur during active braking rather than when friction ceases completely.