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Opeler
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Discussion Starter · #1 ·
Searched but can't seem to find a thread on this topic. Ok so I've got the blue roadster driving but when I start the engine the brake booster pulls the pedal down. I found that I forgot to tighten the two nuts securing the MC to the booster and fixed that. Problem was better but when I start to press the brake pedal the booster takes over and then pulls the pedal down. The pedal does come back up after about 2-3 seconds but definitely not the way it is supposed to work. Other forums suggest bad booster but it was working fine just before the restoration of the car a year ago. Thoughts?
 

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Opeler
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638 Posts
Discussion Starter · #5 ·
Check valve is oriented correctly such that manifold pressure draws vacuum in the booster. Interesting check is that if I flip it the other way thus eliminating the booster the brakes work fine just with zero assistance and all leg power......a lot I might add!

Keith, it is the original GT booster. Was working fine just before the restoration. So if its bad I'm curious what failure mode would cause such a symptom? really baffles me. Is there some form of regulating system inside the booster that allows just enough boost but returns the pedal to the top once foot pressure is released?

Thanks everyone for the help here!
 

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Senior Contributor
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1,642 Posts
I think when the master cylinder decides to Go, it leaks brake fluid out the back /Rear seal and ends up in the booster.
Over time the Brake fluid eats up the rubber Diapham inside the booster and eventually causes a Hole in the rubber bellows, and failure to maintain vacuum boost. In the past, I would remove the MC 2 bolts and try and get a long Cotton Swap as a dip stick to comfirm fluid inside booster,, if it's been off the car exposed to air, might ate hole faster or seized a rusted the spring..... That's my theory :)
 

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Opel Key Master
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5,229 Posts
I’ve had a rebuilt booster in just recent restoration of a 68 Torino that we reused after restoration and it was causing the pedal to stick like you are suggesting. Sent the booster out as that was the diagnosis through troubleshooting, just was bad. Could have developed a leak in the diaphragm, who knows, but not surprising that an original booster Would go bad
 

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498 Posts
I have never had an Opel brake booster apart. I have had plenty of GM boosters apart. The COUNTER you asked about is a large coil spring. I keeps the vac diaphragm from collapsing all the way. That spring could have broken. Never seen one break, but that doesn't mean it can't happen. Suspension coil springs break from time to time, as to leaf springs. No reason a thinner wire coil spring couldn't break, causing what you describe.
I seriously doubt it is a hole in the diaphragm. That would cause the opposite effect: it wouldn't be pulling on the master at all, because it can't build any vacuum.
 

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Opeler
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638 Posts
Discussion Starter · #10 ·
Thanks everyone, this helps and I agree about the diaphragm not being the culprit. That would just cause the booster to be ineffective and worse create a vacuum leak to the intake manifold. Sounds like maybe that spring. I'll replace the booster and maybe take apart the old one to investigate.
 

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Detroit,where my home was
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2,970 Posts
I don't know if you know this but to check the booster you push the brake and then start the car if the pedal pushes further in a bit the booster is ok
 

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And be careful taking it apart. Not a difficult job to do... if you have the right fixture. Not sure how stiff that spring is on an Opel booster. A 60s GM booster can break your nose when you spit the two halves. Make sure there is some restraint.
 

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Opeler
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638 Posts
Discussion Starter · #14 ·
Replaced the booster with a good spare I had and brakes work great, just as they are supposed to do. Before I installed it I checked it over thoroughly, blew out any dust from the inside, cleaned the outside, primed and painted. I can tell you I was able to manipulate the MC out and away from the booster enough to replace the booster and not crack open the hydraulic lines but it was risky! The reservoir had to be twisted 90 degrees causing the connecting tube to come out and thus the lesson learned is use a syringe to drain the thing first. Went back in without too much difficulty and pulled the MC back into place and tightened all the nuts.

Now I haven't taken apart the old booster and probably won't get around to it as I don't have time but I did observe that the connecting rod that extends into the MC was very stiff and didn't seem to move freely like it should. The return spring was intact though and could be seen through the opening. So even though the booster worked a year ago my guess is that fluid probably was leaking and as long as it was getting used it stayed relatively free. Once stored for a year corrosion took over internally and it's dead.

So the moral is that if your brake pedal gets sucked down by the booster when the engine is running or is really slow to return once pressed, the booster is bad!
 

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Opel Addicts
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2,472 Posts
Replaced the booster with a good spare I had and brakes work great, just as they are supposed to do. Before I installed it I checked it over thoroughly, blew out any dust from the inside, cleaned the outside, primed and painted. I can tell you I was able to manipulate the MC out and away from the booster enough to replace the booster and not crack open the hydraulic lines but it was risky! The reservoir had to be twisted 90 degrees causing the connecting tube to come out and thus the lesson learned is use a syringe to drain the thing first. Went back in without too much difficulty and pulled the MC back into place and tightened all the nuts.

Now I haven't taken apart the old booster and probably won't get around to it as I don't have time but I did observe that the connecting rod that extends into the MC was very stiff and didn't seem to move freely like it should. The return spring was intact though and could be seen through the opening. So even though the booster worked a year ago my guess is that fluid probably was leaking and as long as it was getting used it stayed relatively free. Once stored for a year corrosion took over internally and it's dead.

So the moral is that if your brake pedal gets sucked down by the booster when the engine is running or is really slow to return once pressed, the booster is bad!
Wow Mike - I never had heard of that problem before! See you in Carlisle!
 
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