Help with 2005 SL500 hardtop conv. Brakes

[NancyAnne]

Every time I try to brake, the brakes are hard...so I have to push hard to even get the pedal to the floor to switch to “reverse”, “drive” etc.
And then the “Brake” warning goes off and says I have
Hardly any brake power and to push the pedal to the floor to stop
Does anyone have any ideas on what this could be? It’s driving me crazy! Especially when someone pulls out in front of me and I have to hit the brakes quick which sets off the hard brake pedal and warning
Could it be the master cylinder? I appreciate any and all help!

[97 SL320]

Please post the results of the fix to help others. This will be a dealer thing, there was a recently issued extended warranty on certain power brake parts.