Help with brakes

[R5335]

Mark, I hadn't seen anything on this since you last wrote on 11/10/24. So I too have just run into this issue. I'm hoping you found an answer.

My story is similar. The original dual master cylinder was leaking and some fluid was getting into the booster. So I replaced the master cylinder. However, my mistake, I extended the booster push rod to what the manual said to 1.25 inches. Didn't take long to burn up the calipers. So replaced the calipers, had the rotors turned. Reset the push rod from the booster to match the new master cylinder, in my case 1" (actually 31/32). Then off I went and for a few rides ok. Shortly thereafter, pedal was very hard to push and brake the car. OK, videos and discussions with folks says it's a bad booster. So, sent that off to have it rebuilt. Report back was yes brake fluid got in and diaphragm was cracked and the wrong check valve. So, finally got that back. Put it all together, bench bled master cylinder, bleed the system 3 times. Now when the engine is off, brakes are hard to push as would be expected. However, like you, now when I turn the engine on, brake pedal goes to the floor! On top of that, the engine stumbles, loses some vacuum, about 5" Hg. Bleed brakes again, no change. Talking to the fellow who rebuilt the booster says it's a bad master cylinder. Damn thing is only a few hours old. I'm using DOT 3 fluid if that means anything. I'm at a loss as to what the issue is hence why I read your forum with great interest. It also appears when doing a search on the AOIA and SDC forums, this has been a familiar problem with a fair number of people.