That sounds 100% normal. When the brake pedal is up, there's engine vacuum on both side of the brake booster diaphragm. When you apply the pedal, atmospheric pressure (air) is metered into the backside of the diaphragm to create an imbalance, causing pedal assist (pressure boost, hence the name booster). As soon as you release the pedal, that air is drawn into the intake manifold. Pumping the brake is exactly the same thing as creating a pulsed vacuum leak...and more air is basically a larger throttle.

Try the same test with a Chevy or a Hyundai, you'll get the same result.