You're a few steps ahead of most people. Try releasing fluid pressure right at the master cylinder. If that works, suspect the brake fluid is contaminated with petroleum product or the brake light switch is holding the brake pedal down a little.
Fluid contaminated with engine oil, power steering fluid, or transmission fluid will cause the rubber seals in the master cylinder to swell and grow past the fluid return ports, blocking them off and trapping the fluid. The ONLY proper fix for that is to replace every part that has a rubber part that contacts the brake fluid, and flushing and drying the steel lines. That includes rear wheel cylinders, calipers, ABS hydraulic controller, master cylinder, and all four rubber flex hoses.
After you replaced the master cylinder, contamination will leach out of other rubber parts and recontaminate it. That will take about a week or two to show up again, just like you described.
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Friday, March 5th, 2021 AT 11:00 AM