2007 Prius 56,000 miles, master brake warning light on and off

Purchased the car several months ago and have done all maintenance and the car is overall flawless but I believe it to contain the original 12V battery.

3 days ago the brake master warning light came on the dash. No other lights have come on. My scan gauge shows ready and no codes. I checked the reservoir level and seeing that it may have been a tiny bit low, I added fluid and drove the car. Over the last 2 days and about 30 miles the light has come on and off. After adding the brake fluid and taking a few hard bends, the light as I though and issue was gone for good. I even jumped into Maintenance mode. The 12V battery was hovering at 12.1V with the car off. With a load on the battery dropped it down to 11.6-11.8V. The battery level jumped to 14.1V with the car running. I have another 2007 with 100k that I ran in maintenance mode that scored even lower voltage so I figured since I had no battery issues with either car up to this point then the problem with my other Prius have been the reservoir level or sensor.

Well,

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The car sat yesterday and this afternoon (its 60 degrees here) the car is completely dead. My jumper box is very low but I gave it a try and still no start or power but when my wife gets home with the other car, I will try to jump start it.....

Any thoughts? Or is it a no brainer battery?

 

I totally agree, but you also think its a slam dunk 12V replacement issue?

 

Thank You