Brake issues

My brakes had periodically been going to the floor, when I was in stop and go traffic, but would build pressure back when I was able to run longer distances. Yesterday this happened, but never got pressure back. I replaced the master cylinder and bled the brakes numerous times, each time having good pressure when done. When I start the truck up, I lose all brake pressure and the pedal goes to the floor. I have no brakes at that time, but the emergency brake works. Not sure where to go from here. Should I replace the booster?

 

When you replaced the MC did you check for vacuum leaks? Do your brakes work when they get to the floor?

 

I did not check the vacuum hose from the air intake to the power brake booster yet. It currently does not have any stopping power. Brake pedal just goes to the floor. Moved the truck about 20 feet toward and backwards, having to use the emergency brake to stop. I have pressure bled the brakes 3 times with the truck off, and get good pressure when done, but as soon as I start the engine, I lose all pressure and it goes to the floor.

 

Is there a woosh noise and are you losing brake fluid?

 

With the truck engine running in park, I can press the brake pedal down and hear what might be a woosh sound coming from where the pedal arm presses into the booster, but I dont know if a normal sound, or if I'm losing vacuum from the diaphragm inside.

 

A grunt is a sign of a bad MC seal, but lube your pedal mechanics, just in case.

 

The seal in the mc itself, or where it seats into the booster? Again, it is a brand new MC, but wouldn't be the first time I've seen a bad part out of the box. Everything I've read up to this point says dont assume the MC is bad. To try bleeding the brakes again. I've push half of a 32 oz bottle of brake fluid through them, so I would believe I've bled them enough. I just dont understand why I lose the pedal pressure when the engine runs. I'm leaning toward either bad vacuum or brake booster itself.

 

Keep trying. Grandpas trucks symtoms were a honk and dragging brakes. I did MC, hardware, and pads/rotors (glazed). Brakes are peachy.
If you arent pushing brake fluid into the booster, the booster isnt bad. While its running hit everything with carb cleaner. Gently, dont wanna short wires, but listen for the idle to change. Should rev up unless your super close to the throttle body.

 

booster leak make the pedal firmer, it's the master that's bad, or air in system

 

I'm confused on the carb cleaner statement. What does that have to do with the brake pressure? Do I need to let the truck run to build up the vacuum? Did I release a vacuum seal on the booster when I removed and replaced the master cylinder?

 

Proper running system makes its own vacuum, but a leak can cause problems. And the symptoms are close to the problems.

 

There is only the one vacuum hose to the system, correct? The one that comes from the intake to the booster? If there is a vacuum issue would it be the hose or could it be something else?

 

Bleeding the system, did you bleed the load sensing portioning valve on the driver side of the frame rail.

Its attached to the axle with a long rod, air may be trapped up there, once that’s for sure bled, rebleed the rear wheel cylinders

 

Vacuum is a system, it could fail anywhere.

 

The booster gets negative pressure, correct? The intake pulls a vacuum on the booster?

 

Yes, but every gasket has a pressure differential. Diag is a little easier than full guess, but a pedal push is sending fluid somewhere. *super star: i dont 100% know what im talking about, because everything bled and i timed actuation on jackstands. It just worked, but i can only speak for myself.

my next attention would be the wheel cylinders also helpful to do wheels off in 4wd on jackstands. After you clean something the leak shows up.