98 f150 4x4 4.6.....I have a really bad soft pedal. i can step on my brakes with all my might and it will at like ur pushing a quarter of the way. through out the travel of the pedal the braking power stays the same and does not gradually give u more. i had glazed rotors and pads so i replaced them. i replaced the master cylinder and ABS sensors for kicks and giggles even though there was no abs light on. i Even switched from drum to disc in the back in hope of some type of change. AND?????? NOTHING!!! oh i forgot to add i have blead the system a MILLION times. any pointers?

 

Sounds like the booster to me.

 

These trucks have brake lines that get weak over time (usually early models) and replacing them will fix the problem more than likley.Its the short rubber lines that run from the frame to the brake caliper....

 

Just a thought, I know the flex hoses from the frame to the wheels sometimes collapse, I found it out when my brakes (97 2wd) wouldn't release all the way. It might be worth a look.

 

You say you've replaced the master cylinder and converted to rear disc. Did you use a master cylinder for the trucks with 4 wheel disc or 2 wheel disc? Rear disc brakes was not an option until 1999.

 

just using the 2 wheel disc brake M.C.

 

That is most likely the problem, disk brakes require more PSI than drum. By switching the rear to disk and not the master cylinder you are not able to squeeze the rear disc hard enough. When I upgraded my 67 Cougar to disk I had to replace the MC. I believe that they said that drum brakes require 800 PSI and Disk require 12 or 1400 PSI.

 

oh wow that a big psi difference. ok ill try swapping the MC and see if that does anything. ill get back to u guys about it. might be a couple weeks