Is it really necessary to flush brakes when it reaches 3%??? on 2006 Mercedes-Benz E320 CDI

Dealer states that when the brakes reach 3 % fluid they should be flushed. I'm doubting this advice. Can you advise me.

Asked by for the 2006 Mercedes-Benz E320 CDI
I'm not even sure what that means.

You should change your brake fluid at least every two years. Check and see what it says in your owner's manual about mileage. Flushing with a machine is never recommended, but changing the fluid regularly is highly recommended.