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Possible culprits:
Brake pedal push rod needs adjusting (too long).
Recuperation port blocked in master cylinder.
Pedal pivot tight, preventing pedal from returning fully.
Pedal/pushrod pivot tight, preventing pedal from returning fully.
Collapsed flexy hose.
Master cylinder just plain clogged up.
If flexy hose, each hose would affect different wheels.
Can you lift the pedal with your toe? Does it return smartly due to it's own spring?
It all points to fluid not returning to the master cylinder. the fluid gets trapped in the system and expands as it gets hotter the dragging brakes make it hotter and causes it to put the brakes on harder until you can't move. Once cooled down they will ease off and the cycle can start again.
Is the push rod a nice loose rattling fit?
Mart.