i had this issue on my 97 civic.
when your cylinders are ok,
there is only one more thing causing the extreme force, its the fingers on the pressure plate. the spring gets hard over time. hard spring will also cause cylinder seals to leak as well.
since the clutch in my car had done 240,000 kms, it was replaced, complete pressure plate, friction disc, throw out bearing and pilot bearing. all Ok now.