Sticky brakes
 

Hi there!
Hoping someone might be able to point me in the right direction. Bought a 2007 mini cooper base with 6 speed. It has the 1.6 DOHC. The car was purchased from an older gentleman who had developed some knee trouble and was no longer able to drive a standard transmission. The car has 111k on it but seems like it was taken care of. The Mini test drove great and made it 25 miles to my home just fine. Took the car out for a short road trip that ended with 2 frozen rear brake calipers(flatbed tow to get home). The car was not used much for the past year by the previous owner. I assumed the calipers were cruddy and replaced them today, also flushed out all the cruddy old fluid during the bleed. The car did great until I had to brake hard. Once I brake hard one or more calipers still freeze closed, this is not limited to the new rear calipers, the front sometimes hang as well. Once the brakes freeze the only way to unstick them is to bleed them out a little again. Seems like the calipers and fluid may not be the issue here. Any help or guidance would be greatly appreciated.
Thanks,
-Jeff