Brakes slowly start to lock up. After time, they release slowly?

If I leave my car sit for hours, the brakes will feel normal. If I drive and use the brakes, eventually the brakes "lock up" (I believe because of the brake fluid not being released from the lines) Would this be a master cylinder malfunction? Or could something be stuck in my lines?
Any estimate on a repair cost? Should I just take it to a shop (I'd hate to be told I have to replace lots of brake parts if it could be solved with a brake system drain or something simple)
The vehicle is a 2001 Honda Civic EX (with ABS)
Any help is appreciated!
Thanks!

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