1989 mercedes 300se check engine light on when brakes applied
 

I have a 89 300se with 89000 miles. my check engine light just came on .If I start the car and it is in park running, the check engine light is off , once I touch the brake pedal, then the light comes on and stays on? only comes on when I apply the brake pedal, then remain on until I shut her down. anyone encounter this before? Brake fluid up to level