If you are sure there is no air in the system and there are no leaks anywhere, the most likely culprit according to your symptoms would be the master cylinder. You may want to swap it out and see if there is a difference.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record