2003 F350 4x4 5.4L auto, the brakes are usually right there at the top when I touch the pedal, yesterday it started acting funny by the brakes not engaging until about half pedal, it was just like pressing on air then i have brakes. I checked, I have no fluid leaks and I don't hear or see any vacuum leaks (cracked hose). Today on the way to work, the brakes were fine for about 10 miles, then they started doing it again, this time they went almost to the floor before the brakes applied, I pull into work parking lot put in park, I have full brake, put into reverse, back up a little, full brake, put in drive, pull forward a little, full brake, It's almost like lower rpms and brakes work, higher rpms and no brakes. Any thought's would be appreciated.

 

Update: On way home today pedal was consistently going to floor if I pressed normal, If I pressed quick and hard I had full pedal, as long as I kept pressure applied, If I let off the slightest the brakes went to the floor. Got home parked and now even with truck not running if I press slowly the pedal goes to the floor. I'm thinking the MC took a dump but how do I verify that? I don't want to just start throwing money and see what fixes it.I could also be the ABS unit from what I've read and I see those run about $600.

 

likely the master brake cylinder is failing

 

I replaced the master cylinder and as soon as I hit the brakes the left rear brake line blew out. I replaced that and all is good. I'm glad it happened while around home, I've been to Mississippi, 2times in the last 2 months and this last time I was pulling my enclosed trailer, it could have been very ugly if the brakes decided to fail then.