1992 Honda Accord Brake pedal goes to floor board when AC i

When we use the AC the brake pedal goes to the floor board ever so slowly. When the AC is off the brake pedal works just fine and the brakes work fine. I even try to pump the pedal when the AC is on and it slowly goes to the floor board but the brakes do activate and stop the car. What could be causing the problem? Is it the brake cylinder? Why is it only when the AC is activated?
thanks for the Help,
sincerely,
Todd