2003 Peugeot 406 foot brake

I can't find a complete listing for your car so I can't tell which type of power brake assist you have. If you have a typical vacuum power booster, check the hose connecting it to the intake manifold. If it's disconnected, the huge vacuum leak will cause engine speed to increase with no corresponding increase in power, unless you have a diesel engine, and with no vacuum to the power brake booster, you will still have brakes, but no power assist. You will have to push really hard on the brake pedal.

my peugeot 406 is a diesel model

Diesels are wide-open to air and don't have a throttle blade like on gas engines, so a vacuum leak, no matter how big, won't be noticed in engine performance, but it still could cause a power brake problem. What you also need to look at is many vehicles with diesel engines don't use vacuum-operated brake boosters because the vacuum source is not reliable for this purpose. You could have a separate vacuum pump just for the brakes, or there could be a boost system run with power steering fluid. I can't find any information on the brake system for your vehicle so the best I can suggest is to have your mechanic start with a visual inspection under the hood.