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Brake Booster Issue

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Blue68SST View Drop Down
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    Posted: May/08/2021 at 2:38pm
Two years ago I replaced the master cylinder and brake booster on my 68 AMX. All was well, until this week when after about 45 minutes of city driving, the brake pedal went hard, and almost all braking was lost. Luckily, I was around the corner from home, and made it safely. 
I checked the brake fluid, full, and none of the wheels were overly warm.
After about a half hour, I re-started it and braking returned to normal. 
It happened again today, and I pulled over, turned the engine off, restarted immediately, and again, all was normal.
I checked the vacuum line, its good, and is pulling 14 lbs on a vacuum gauge. 
I am running an Offenhauser dual quad, with twin Carter AFB'S, the brake vacuum line is running off the rear carb.(390 4 spd)
Could it be the check valve that is attached to the booster?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: May/08/2021 at 4:34pm
Make sure the pivot links between the booster and firewall aren't binding
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: May/08/2021 at 4:41pm
If the booster goes bad, the pedal goes soft and the braking is poor. A car that has not been used can have a booster seize up.
Inclined to believe the master cylinder has a bad bore or contamination that has the piston sticking.
I'd flush the brake fluid first and see how the fluid looks. If it's clear, then I'd look at rebuilding the master cylinder.
Also inspect the brake pedal linkages. If you loose a cotter pin, a pin may be part way out and be randomly jamming on a bracket. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blue68SST Quote  Post ReplyReply Direct Link To This Post Posted: May/08/2021 at 4:52pm
I find it odd that that by just turning the engine off and on again, it returns to normal, which is why I thought it might be a vacuum issue?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: May/08/2021 at 5:04pm
Have to agree it seems odd.
That stated, if everything else looks good, try pulling the master cylinder off the booster and inspecting the pin. A broken pin or worn pin mount in the booster may let the pin occasionally go off center.
I'm stretching here as a broken or worn pin mount is unlikely, but you do state city driving.
Likely no binding issues from corrosion and more likely something has worn out or broken.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: May/08/2021 at 8:05pm
I'm guessing you have a booster problem.  I had virtually the same thing happen to a Dodge pickup with power brakes.  Normal, then took both feet to stop.  Turn off engine, normal, then back to both feet.  Replaced booster (was under warranty) and problem went away.

If you think about how a booster works, when you depress the pedal the internal valve vents the back of the booster.  If the valve hangs up, the booster is pretty much immovable with vacuum on both sides of the diaphragm.


Edited by 6PakBee - May/08/2021 at 8:08pm
Roger Gazur
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1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blue68SST Quote  Post ReplyReply Direct Link To This Post Posted: May/08/2021 at 9:01pm
I had a really hard time finding the booster 2 years ago, but it seems the most likely culprit. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blue68SST Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2021 at 9:24pm
Well, problem solved.
Not a faulty master cylinder, nor a faulty booster, or even a bad check valve.
A bad vacuum hose.
It got soft after it warmed up, then collapsed, closing off the vacuum to the booster.
When the engine shut off, it returned to shape, at least temporarily, and was good to go.
$2 fix.
Bam.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Goodtimes Quote  Post ReplyReply Direct Link To This Post Posted: Jun/03/2021 at 8:51am
Good detective work, who would have thought, usually they get hard and crack
69 AMX 290 4spd leather
68 AMX 390 A/T 70 front
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Jun/03/2021 at 9:00am
Now that's a new one on me.  Usually the hose turns hard as a rock from heat.  In any case, glad you found the problem. Thumbs Up
Roger Gazur
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1970 Sonic Silver auto AMX

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