Scotty,
I have a 98' F150 with the 4.2L V6 with 153,000 miles on it. Overall the truck runs great for its age but the brake pedal is very spongy and sinks pretty far down every time I need to brake. I replaced the master cylinder thinking that it was bleeding internally but that didn't seem to fix it (I also bled the brakes from the master cylinder after I installed it). Everything I read online says a low brake pedal means the brakes need to be bled but I did that when I put the new master cylinder in. As a regular DIY guy I'm really struggling to figure out what the problem is. It's also worth noting that my truck isn't leaking brake fluid, I've looked everywhere. Any suggestions?
-Nic
Did you bench bleed the master cylinder before installation? Another possibility would be a 24-year-old rubber hose ballooning under pressure.
Additionally, if the truck has ABS you may need to use a bidirectional scan tool to properly bleed the brake system.
In addition to what @chucktobias says, if you for sure did everything correctly and your pedal still sinks, the new master cylinder could be bad. I went through 3 different cylinders last year and a whole lot of frustration replacing that in my 1979 Pontiac because of bad parts.
