The RPMs lower when the brakes are applied.
When a vehicle is running all of the accessories and systems used to run the motor all put load on the system. This being said, anything from turning on your headlights to depressing your brake pedal puts extra load on the engine. The modern brake system can use a vacuum booster system to provide a nice smooth brake pedal feel. It does this by using vacuum supplied by the motor while running. When applying the brakes, the vacuum booster uses supplied vacuum from the motor to provide a easy brake application. Doing so puts a load on the motor causing a slight drop in rpm. This is a normal response. Unless there is already a large load on the motor causing the added load from the brake booster when the brakes are applied that the engine starts to stall or smoke. The only way to find out whether this is normal or if there is an issue would be to have the vehicle inspected by a certified mechanic.
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