ive had this problem for a bit when my truck is off the pedal is stiff, nice n solid pedal but when you turn the truck on the pedal likes to go all the way to the floor and i just put 450 dollars into just the front brakes, new everything, pads rotors and calipers. I bled the brakes and it was even better than before the pedal barelly even moved from the top it was nice and solid, but still when i start up the truck it doesnt hold the pressure like its leaking through somewhere, it will stop like normal when you start to hit the brake but when you want to stop faster and push down hard the pedal allmost hits the floor and doesnt even feel like its trying to stop.. Can it be a check valve? can the pressure leak through the cap ontop of the master cylinder resevoir? i really need some information on this..thanks

 

Did you bleed the rear brakes to? and did you replace the rear at the same time as the front brakes?

 

i replaced them before the front because i replaced the front pads and the rear brake shoes n hardware and bled them multiple times after that.

 

the booster pump might be shot.