[Miata] Brake problem

[Michael Muth]

I had a problem this morning when I first started the car and backed out of 
the driveway where the brake pedal pressure was very hard to push. I took my 
foot off the brake and pressed again and it seemed normal. Drove for a bit, 
tried the brakes and they seemed to work fine. Short distance, I hit the 
pedal, worked fine, pressed again and got this hard backpressure again, 
almost like I lost the power assist. Pulled over, checked under the hood and 
everything looked ok. Occurred one more time, then normal? Any ideas? The 
weather has bottomed out so it was below freezing all night. Car was parked 
in a garage all night. Master cylinder?

Mike-in Southern maryland