brake pedal goes to the floor at start up

[73dartneedswork]

With my foot on the pedal, I have a good strong brake while my motor is off. As soon I as I start the engine the brake pedal goes to the floor. I’ve replaced the master cylinder (bench bled it), and have bled the brakes three times. My calipers are on the correct side. I don’t see any leaking fluid at the wheel cylinders or calipers. I do have power brakes. Is there any way the booster could be the problem? Any ideas?

Thank,
Kris

 

[deman360]

probably the booster.

 

[yellowdartdave]

Yep, bad booster.

 

[green1]

Put a HF hand vac on it, and see if it bleeds down.

 

[73dartneedswork]

I replaced the vacuum boster and it did not correct the problem.

 

[6pk2goDemon]

Did you buy a new or rebuilt M/C unit?

You can isolate the M/C by clamping off the 3 brake hoses using rags or towels in the jaws of some vise grips so as not to damage the hoses. If the pedal drops w/ the hoses clamped closed, it's the master most of the time.......

Best I can do to help. Good luck, brakes can sometimes be a PITA.

 

[dlisabeth]

Did you ever find a solution to this problem? I am having the same exact problem with a truck that I am building and it is driving me crazy.

Thanks,

Derek

 

[green1]

Correct push rod?

 

[dlisabeth]

I fixed my problem this weekend. It ended up being a master cylinder that was too small and couldn't push enough fluid. I upgraded to an mc with a larger bore and it works perfectly.

 

[RustyDusty]

Did you buy a new or rebuilt M/C unit?

You can isolate the M/C by clamping off the 3 brake hoses using rags or towels in the jaws of some vise grips so as not to damage the hoses. If the pedal drops w/ the hoses clamped closed, it's the master most of the time.......

Best I can do to help. Good luck, brakes can sometimes be a PITA.

Don't ever clamp brake hoses!!! It breaks them down on the INSIDE, not the outside. Putting a cloth over it doesn't do squat. They will turn into a one way check valve if you do that enough.