i know this is a common thread. i have been searching for the last 2 hours trying to figure out what is going on. when i push the brake pedal in i get nothing. it goes straight too the floor so i went through process of elimination. i have adjusted the rears, replaced the booster and the master and still have the same problem. it does not want to build pressure. it will if the truck is off and i pump the brakes. but as soon as i start it it goes straight back to the floor. i have bled the crap out of the system. also the abs has been unplugged and bypassed in the rear. could it be a vacccum isuuse? the cruise and heater controls all still work so i am kinda stumped?

 

Vacuum assists in braking, loss of vacuum should make the pedal stiffer.. it sounds like the symptoms of an air bubble in the system or a loss of hydraulic pressure..

 

Id check the rubber lines, you might have a leak somewhere

 

I will try to check some more stuff. I'm gonna buy 2 plugs for the master and undo the lines and plug them and try to eliminate the mater and brake booster

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You are either losing fluid, ballooning a hose, or have an air bubble in it. I'm assuming the master cylinder you've installed is good. (DOA's do occur)

The rear brakes have to be bled by the book, or they'll trap air.

 

On occasion my brake pedal will go right to the floor with no takes,but it's usually after a back and forth session with the steering wheel or a full lock turn.