If they are bled properly, I would go with the master cylinder as a possible culprit. When you hold the brake pedal down, does it slowly sink to the floor, or does it hold steady in the same position?

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Ali Al-Chalabi

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