Break problem Porsche Cayenne Turbo 08

I would guess something to do with the brake booster from your description of a hard vs soft pedal. The reasoning behind it isn't crystal clear however. Also, it does make sense that there'd be no lights or alarms as most brake boosters are entirely vacuum controlled, and because of the variance in vacuum in a typical drive cycle, I'm sure most manufacturers wouldn't want false alarms. Do you have ANY type of check engine light on or anything that would indicate a vacuum leak? If not, it could also be as simple as the booster check valve being bad. This also goes on the assumption that you aren't hearing any hissing noises that are abnormal. Be sure not to drive the vehicle until this is repaired/solved. Your Cayenne is a VERY heavy vehicle and you absolutely can not reasonably muster enough force on the brake pedal to stop in an emergency situation with a malfunctioning brake booster or other brake system problem.

There is no check engine and no alarm

So, you need to have a mechanic inspect the brake system, probably focusing on the brake booster and/or the engine's vacuum system. Again, be sure the vehicle is towed to where ever you plan to have it repaired.

The brake booster is bad. Having to step hard on the brake pedal is a good indication of a bad brake booster.

Same issue with my 2005 Cayenne S. Pedal went to floor, provided only minimal braking. Fluid reservoir is just under MAX line.

Check the vacuum lines for the breaking system. It’s not the brake booster. I had the same problem with my 06 Porsche Cayenne, changed booster and until I check the vacuum hoses and make repairs it worked