When I push down the brake pedal as far as it will go w/o using excessive force, the truck stops, but then the brake pedal will slowly start to begin going lower and lower. I have to pump the break to get the pedal back to where it feel like I pressing on the break.

What does this mean, and how do you fix it?

Thanks

 

Sounds like you've got a brake fluid leak.

 

Usually means your master cylinder is going

 

yup. . i have the same issue in mt 1988 ford f350 right now. just a case of a worn out master cylinder. super easy to fix. two bolts and two lines. slip on the new one and done