I had a similar occuance of the brakes binding on and the pedal being really hard many years ago on a Spartan Treka. One telling thing was when releasing a bleed nipple I got a brief squirt of brake fluid, indicating it was caused by hydraulic and not mechanical reasons.
In my case I had a small leak from the master cylinder that was stopping the piston returning fully. After a few minutes the leak would drain into the servo and the pedal work again.
Just a thought.
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