I have this problem when applying the brakes: It feels like " bump, bump, bump " in the brake pedal and slows down as the truck slows ( 2005 F150 Super cab 4X4 ). I put new rotors and pads on the front, thinking that was probably the problem, but it didn't fix it. My next idea is to rotate the tires ? I put 33 X 12.5 tires on last year with a 2 1/2" leveling kit. They are about shot now, as my upper ball joints are bad, maybe due to the larger tires. Not sure what this could be.
Any ideas?

 

as in a pulsing or wobbling? if so that may be warped rotors. its odd that its still happening after you replaced the fronts though. i dont think ive ever seen rears warp

 

are you sure its not ABS activating? Do you know what that feels like?

Its probably either ABS or warped rotors.

 

I'm running into the same issue with the 99 I just bought. Previous owner had the front brakes gone through and were given the thumbs up but I get a soft pedal and a rhythmic pulsing that really seems like either ABS or a warped rotor or drum. Haven't had the chance to pull the tires and take a look yet but its the first thing I have to run down because its annoying as all hell.