Commosmart1

I have a 1984 f150 with 302v8

I changed the front disc brakes, calipers, rotors, and the master cylinder. I bled the new mc and installed it then went to bleed each caliper bleeder and i cant because the brake pedal is sunk to the floor. When the truck is not running i have firm brake pedal as soon as i crank the truck the pedal sinks to the floor. This truck has a brake booster i have checked everything and cant seem to figure this out please help

Commosmart1

When i block off the booster vacuum hose my brakes work

Purplehazeir

Sounds like the booster might not be receiving vacuum from the engine, possibly due to a plugged vacuum hose, a vacuum leak or a defective check valve. It can also indicate a failure of the booster itself.

Commosmart1

I think its gettin vacuum because when i take the vaccum hose loose from the booster the truck dies. I thought if the booster fails the pedal is hard to push down?

Purplehazeir

To test for vacuum problems, start the engine and run it to medium speed, then turn off ignition and take your foot off the gas. Wait about a minute and a half and then apply your brakes. You should feel the vacuum boost on at least two brake applications. If you do not, you may have a vacuum system leak or bad vacuum check valve.