So here's the story. My (baby my) 2001, 1500 4x4 is having braking issues. I can slam on the brakes and have full stopping force. However, when if I let up on the pedal 3/4 of the way an re-apply im to the floor boards. At low RPMs it have no real braking force. I am thinking this is my master cylinder telling me to change it out. Any suggestions? I have change out the power booster check valve, thinking it was a vacuum issue.

 

if it were a vacuum issue your pedal wouldnt go right to the floor...you would just loose the power assist on the brakes just makeing it harder to push the pedal to come to a stop..im leaning towards you have a small leak somewhere in the system go over all your lines...i had the same thing happen to my girlfriends car turned out to be a small pinhole in a rubber line..

 

I am pretty sure you have a bad power booster.

 

Pedal going to the floor is a hydraulic problem. Vacuum booster just helps you push the pedal, doesn't control pedal height at all.

Need to check fluid level, check for leaks, or evidence of a past leak... (brake fluid running down the front of the booster is a good indication of a failed M/C.)