Ok lets try this again, I have this problem with my brakes on my 2000 superduty 4x4
when you go to brake the pedal is very hard to push, and It feels like the pedal is chattering the brakes don't work, but there is no noise such as a chatter, so i let up on the pedal and push it again and most of the time the truck brakes work just fine.
It happens more often in the mornings or when you first start driving the truck.
The ABS light came on ther other day for 15 seconds then off, this has happen a couple of times but not latel. note the ABS light has never come on durring the braking problem.
about every 10 - 15 thousand miles I clean and apply anti-siez to the front caliper pins. so Im not sure where to start looking..

 

sounds like you have air in your brake lines maybe... checked your brake fluid level?

 

I thought that too, but it was full, and it doesn't do it all the time. It happed monday morning right after i left the house, then about 5 miles away, I pulled my boat to the shop and back and never had it happen.