I have a 2001 P38 4.6 HSE with just under 125k miles. Recently, after it has been sitting overnight, when I start the truck the brake pedal goes all the way to the floor with no stopping power. If I pump the pedal a couple of times I get my brakes back and they are fine for the rest of the day. There is plenty of fluid in the reservoir and I have no leaks that I can see.

I think I already know the answer. It seems to me that the master cylinder is going bad but have also considered the accumulator. I would appreciate either confirmation or any other ideas that you guys might have as to the reason for my brake issue.

Thanks

Mike