Brakeless bir when tne

[robertop]

1969 Hemi roadrunner with SuperTrack Pack.Recently running on an expressway and slowing to my exit the car sustained a complet brake failure, after few seconds that the dash brake light had momentarily turned on and off. I was able to stop safely and, after a change of underwear,I had the car towed home. Now, with the engine off, the brake pedal is solid, but goes to the floor when the engine is turned on. The fluid in the mc is still full an there are no leaks under the hood or floor. Every component of the system has been rebuilt/replaced through the years and the brakes worked very well when the car was first started. My opinion is: mc or booster failure. Do you guys know how to check for sure?
been rebuilt/replaced and the brakes worked very well when the car was first started. No opinion is this: failed mc or booster. What do you guys think?

 

[Russ69Runner]

Have rebuilt the booster diaphragm on many car's that the same thing happened. Brake pedal to the floor no brake's. You need two people one in the car pushing the brake and another out side the car checking the hose with your hand. If you don't feel any action on the hose with it against the palm of your hand the diaphragm is bad. Or you can here a wishing sound. But the best bet is to rebuild it that is what sound's like is wrong.