Brake Fluid Level

In all my years of driving I've never checked this before but because I just installed a bbk, monitored the brake fluid level a bit more closely.

In the morning when the car is cold, before starting engine, my brake fluid level is about 1cm below max mark. Then after half an hour of driving, I check again and the level would rise to the max mark. Is brake fluid supposed to expand like this? Or do I have an air bubble in the system causing this fluctuation of level?

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