Brake conundrum

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69moredoor

69moredoor

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Got a Brake question that has me chasing my tail…

New master cylinder
Old booster
Lines bled and bled and bled
Check valve in booster checked

My old master was leaking so I replaced it. Brakes were soft at the time

After repairs: have firm great feeling brakes when car is off
Start car…they get super soft and just grab by the floor.

Thoughts?
 
Check the master cylinder actuator rod. Sounds like the replacement master cylinder differs from the old one and the rod is now too short. I've seen it a good bit.
 
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Did you bleed the brakes with the engine off? Engine can't be running when you bleed the system.
If that's good, then it's a bad booster. If your bad master cylinder leaked ANY fluid into the booster, it's gonna go bad. And the symptoms are similar to what you describe.
If you don't believe me, then listen to Booster Steve. Around 4:50, but the whole video is worth watching.
 
Professor Fate said:
That doesn't work on power brakes.
Click to expand...
Then somebody needs to tell that to Ford Motor Company. Every older Ford I've ever put a master cylinder on that had power brakes had an adjustable pushrod between the master cylinder and booster, including the 75 F250 I have right now. I even had to readjust it shorter when I replaced the master cylinder, because the new master cylinder was a little different and held the front brakes slightly applied. When I shortened the pushrod a couple of threads, all was well.
 
RustyRatRod said:
Then somebody needs to tell that to Ford Motor Company. Every older Ford I've ever put a master cylinder on that had power brakes had an adjustable pushrod between the master cylinder and booster, including the 75 F250 I have right now. I even had to readjust it shorter when I replaced the master cylinder, because the new master cylinder was a little different and held the front brakes slightly applied. When I shortened the pushrod a couple of threads, all was well.
Click to expand...
I was referring to the Dr. Diff adjustable pushrod that was mentioned.
Yes, the booster rod has a specific setting in the FSM.
 
Been there, done that...
Air still in the system. Booster operating, it supplies more braking force & it is compressing the air in the system.
 
Doubt if this fault would be bad booster, for two reasons:
- it is 'boosting', applying pressure, with the engine running, as evidenced by the pedal moving.
- modern boosters are designed with an internal arrangement so that they still apply pressure if the vacuum apply section fails.
 
Professor Fate said:
Did you bleed the brakes with the engine off? Engine can't be running when you bleed the system.
Click to expand...
1 huh 1.gif

"Engine can't be running when you bleed the system."

1 no, just no.jpg


I have never heard anyone state this before. Care to edit the comment?
 
Kern Dog said:
View attachment 1716011110
"Engine can't be running when you bleed the system."

View attachment 1716011111

I have never heard anyone state this before. Care to edit the comment?
Click to expand...
Nope. Learned it back in high school shop class.
And as I said, if you scoff at me, refer back to the video I posted. Stated bluntly by Booster Steve, who took over Booster Dewey's business. One of the most respected classic booster rebuilders in the country. Right at about 24:18.
 
Hitting a disagree when I stated that I hadn't heard anyone state the procedure before...AS if you have some way of knowing what I have heard in my life. That is a stupid response on your part.
Your buddy "Booster Steve" says it, then it MUST be common knowledge, huh?
Try starting a poll and see if the majority of the members here follow this procedure or have even heard of it.
I'd be willing to bet that you'll find that whether this procedure is correct or not, less than half of the members here know or follow it.
 
The reason it's not advised to bleed the brakes on a power brake system with the engine running is because the activated booster can act like a "shock absorber" to the brake pedal and not allow it to get its full stroke, thereby leaving air in the system.
 
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RustyRatRod said:
The reason it's not advised to bleed the brakes on a power brake system with the engine running is because the activated boosted can act like a "shock absorber" to the brake pedal and not allow it to get its full stroke, thereby leaving air in the system.
Click to expand...
Man, Rusty, are You Showing Off...... AGAIN? LOL!!!!!!!
Love the Knowledge all Y'all Share! Thank YOU ALL!!!!!!!
 
Kern Dog said:
Hitting a disagree when I stated that I hadn't heard anyone state the procedure before...AS if you have some way of knowing what I have heard in my life. That is a stupid response on your part.
Your buddy "Booster Steve" says it, then it MUST be common knowledge, huh?
Try starting a poll and see if the majority of the members here follow this procedure or have even heard of it.
I'd be willing to bet that you'll find that whether this procedure is correct or not, less than half of the members here know or follow it.
Click to expand...
I'll pull the red x, since you did say you'd never heard of it. It was just a kneejerk response to your snarky pictures and "care to edit?" comment.
 
Not being the Police here, But....as We're going into the Holidays, I re-read the Big Daddy/FMA thread, someone stated "We ain't Chevy Boys, we have no room for infighting, and WE need To stick together" NOT a Direct quote, and None of my Darn Business.... but I Dig It When All of US are in Tune! Peaceful wishes Everyone! And NO, I'M not some damn happy hippie! In all Fairness, I had a myself and a Fleet Maintenance Shop OWNERS Son, ( my old Drummer and his Father, Lil Rob and Bob, both No Longer with us. RIP..they both kicked ***! Robbie was one of The BEST DRUMMERS AND Friends I've ever had. And I've played with 2 signed bands, playing some of the MOST TECHNICAL, and Physical Music ever Written..) now, granted, we were kids, but when we bled em, We went engine on..... BUT I Doubt we Called Bob (Rob Sr) on that for advice....I hate power brakes, steering, auto transmissions, etc almost as much as Leftists! Sorry to Rant. I miss them Guys, and I can see them in my minds eye.......
 
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Professor Fate said:
I'll pull the red x, since you did say you'd never heard of it. It was just a kneejerk response to your snarky pictures and "care to edit?" comment.
Click to expand...
His middle name is Snarky. I thought everybody knew that already. lol
 
Ironracer said:
Man, Rusty, are You Showing Off...... AGAIN? LOL!!!!!!!
Love the Knowledge all Y'all Share! Thank YOU ALL!!!!!!!
Click to expand...
Hardly. With brakes, anything I've "learned" has been through repetition. I've done it so much, it's pathetic. Even a chimp can learn that way. lol
 
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