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Discussion Starter #1
I've been working on getting my Manta A back on the road and we are sooooo close on a mechanical perspective. But something came up yesterday that has me scratching my head.

When you start the car, the brake pedal pulls itself to the floor. And when you accelerate, it comes back up



When you start the car with the foot on the pedal it doesn't move at all (after the initial 1-2" with the car off), since I've already pushed out whatever vacuum pressure the booster was holding. At idle the brakes are lightly applied and the brake lights are on.

I replaced the vacuum hose running from the booster to the manifold and, with the car running, sprayed some starter fluid at each of the junctures where the hose has something in it, i.e. the booser, the check valve, and the manifold. There was no change in rpm at all.

The only place we got a change in RPM was in the MC area, but I put a fat dob of grease on the seal inside to test if it was leaking there and when I did the RPM stopped changing.

Anyone have any thoughts? I think the booster might be bad.
 

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Can you just suck on the vacuum line to the booster? It should hold a vacuum. If not, the diaphragm is blown. I guess you would want to use a vacuum pump instead of placing your lips on the rubber hose and see if it holds.
 

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OK, not going to the floor but moving to engage the brakes.

Control valve in the booster looks like it is stuck 'active' all the time. It should only be active when you put pressure and movement on the brake pedal yourself. Check valve may not be working either; if it was, then this would only go one way: down. Edit: I may not be right on the latter part about the check valve; it might be a slow leak in the diaphragm.

Can you post a pix of the booster? I have 2 in good shape. See my detials in your WTB ad on the tears of fitment.
 

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And just to check: Have you engaged the brakes while rolling the car and the engine not running to see if the brakes are working anywhere close to properly?
 

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I think the Booster has a Hole in it,,, or the Hoses have small leak and slowly regains vacuum ,,
usually when the booster Goes bad.
The Master Cylinder has a Problem with the rear seal,, and the Corrosive Brake Fluid leaks into the Vacuum Booster Can, and eventually makes...Eats ... a Hole in the rubber Bellows inside,, makes vacuum leak
That what happened in my pass experience. with a GT

If the hoses aren't leaking

Maybe you can remove the vacuum hose,@ booster turn out the Booster fitting.
Get a Long Cotton Swap, or make extension... try and snake the swab down to the bottom of booster.
Like a dip stick... If it's wet,with brake fluid ..... the booster is Done,, need new one.. Just my honest opinion, and the Master cylinder too

Good Luck and hope it's only a Leaky...old dried up Vacuum Hose
 

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Discussion Starter #6
All the vacuum hoses are brand new. We pulled the master off to check for evidence of there being fluid behind it at the seal, which there wasn't any.. I've had bad masters before on other cars and this was always the tell-tale sign.

And just to check: Have you engaged the brakes while rolling the car and the engine not running to see if the brakes are working anywhere close to properly?

I have! we drove it around for a while and the brake pedal feels like there is no booster. Hard, but does stop the car eventually. The more worrying issue is that the brakes are slightly applied all the time, so if you just coast, you'll slow down more rapidly than if they weren't.
 

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Discussion Starter #7
OK, not going to the floor but moving to engage the brakes.

Control valve in the booster looks like it is stuck 'active' all the time. It should only be active when you put pressure and movement on the brake pedal yourself. Check valve may not be working either; if it was, then this would only go one way: down. Edit: I may not be right on the latter part about the check valve; it might be a slow leak in the diaphragm.

Can you post a pix of the booster? I have 2 in good shape. See my detials in your WTB ad on the tears of fitment.
I saw your post on the WTB ad! Thank you for that information!! I don't live where the car is stored so I'll see if I can have a friend go and snap a picture of the booster and master for me.

I was looking for. 74/75 booster since they are, from what I understand, a direct fitment on a 74 Manta.
 

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I saw your post on the WTB ad! Thank you for that information!! I don't live where the car is stored so I'll see if I can have a friend go and snap a picture of the booster and master for me.

I was looking for. 74/75 booster since they are, from what I understand, a direct fitment on a 74 Manta.

Check valve Failed
 

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According to my 1977 Buick dealer parts book, the '75 is different from the '74. Looked on my '75 Ascona and it is larger in diameter than the earlier ones I have. They would be from '73 and earlier models of Manta's and Ascona's.

Good that you pulled the MC off and looked! So the input seal is OK. Does not mean that the piston seals are both good.... just keep that in mind. The 'hard to stop' would indicate that is possible.
 

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Keep in mind there are 3 distinct brake boosters for the Manta. Early ones (late 1970-sometime in 1972) are for ATE master cylinders and they have a narrow bolt pattern. The brake pedal pivot is different too.

Then they went to Delco master cylinders with a wider mounting bolt pattern. The booster changed, and when it did the brake pedal arm did too. This was from 1972 thru 1974.

The 1975 Opels retained the Delco master cylinder but got a larger OD brake booster.
 

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Discussion Starter #12
According to my 1977 Buick dealer parts book, the '75 is different from the '74. Looked on my '75 Ascona and it is larger in diameter than the earlier ones I have. They would be from '73 and earlier models of Manta's and Ascona's.

Good that you pulled the MC off and looked! So the input seal is OK. Does not mean that the piston seals are both good.... just keep that in mind. The 'hard to stop' would indicate that is possible.
The 75 had bigger brakes all around from what I've read. But the bolt pattern should be the same as the 72-74 booster. If the MC was bad the pedal would go all the way to the floor as it would be letting fluid out.

I think it's "hard to stop" because the booster isn't functioning properly and I'm suddenly braking in without the help of the booster.
 

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Discussion Starter #13
Keep in mind there are 3 distinct brake boosters for the Manta. Early ones (late 1970-sometime in 1972) are for ATE master cylinders and they have a narrow bolt pattern. The brake pedal pivot is different too.

Then they went to Delco master cylinders with a wider mounting bolt pattern. The booster changed, and when it did the brake pedal arm did too. This was from 1972 thru 1974.

The 1975 Opels retained the Delco master cylinder but got a larger OD brake booster.
I'm not sure which one I have.. but if it's a 74 it should be a Delco correct? It's pictured above.
 

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Well now hold the phone ...... If the MC was bad with the front piston frozen, the pedal would go down less than normal but not quite all the way to the floor. And one half of the braking system would be not properly activating. We just can't interpret 'stops hard' as you are feeling it! Just check the rear drums to be sure there is no fluid leaking back there.

If the pix is of what you have, then I have one of those for sure. I took off the MC like that earlier today. But that booster is the least good of the 3; rust a fair amount on the outside. Holds vacuum for about 5-10 seconds. Inside look OK from what little I can see, except that a spider got in there LOL.

I'll look to see if the other 2 are like that, and let you know. They do look like the booster illustration in the '74 FSM, so that is encouraging. It'll probably be tomorrow.
 

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I'm not sure which one I have.. but if it's a 74 it should be a Delco correct? It's pictured above.
Yes, you have a Delco MC pictured.
 

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Discussion Starter #16
Well now hold the phone ...... If the MC was bad with the front piston frozen, the pedal would go down less than normal but not quite all the way to the floor. And one half of the braking system would be not properly activating. We just can't interpret 'stops hard' as you are feeling it! Just check the rear drums to be sure there is no fluid leaking back there.

If the pix is of what you have, then I have one of those for sure. I took off the MC like that earlier today. But that booster is the least good of the 3; rust a fair amount on the outside. Holds vacuum for about 5-10 seconds. Inside look OK from what little I can see, except that a spider got in there LOL.

I'll look to see if the other 2 are like that, and let you know. They do look like the booster illustration in the '74 FSM, so that is encouraging. It'll probably be tomorrow.
Fair. But, it isn't so much a "stops hard" or that it's "hard to stop" it is hard to push the pedal. The brakes feel like unboosted brakes, but they do work. I just had the front calipers rebuilt, I replaced the shoes and cylinders of the rear drums (didn't see any fluid in there afterwards), and I replaced all the brake lines with brand new rubber units.

What's pictured is the actual booster and MC that are in my car.

How long should a booster hold vacuum?
 

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IDK for sure. I am just putting my finger over the hole on the 1st one, and that may not be a great seal. I'll check the other 2 today and send you a PM.
 

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Discussion Starter #19
A little update:

I don't know why I didn't try this earlier, but I disconnected the vacuum hose from the booster just to see what's going on with no vacuum, and unsurprisingly, the pedal stopped moving on it's own.

What I can't surmise is, if the manifold is constantly drawing vacuum, what is the purpose of the check valve?
And if it's constantly drawing vacuum, then why does the pedal return to normal when the vacuum pressure increases under revs?
 

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The check valve keeps the brake booster in full vacuum, yes it did hold a vacuum after I shut the engine off and removed the hose between the check valve and booster.
My understanding of the check valve is so that your brakes still function after the engine cuts off.
The diaphragm in your booster sounds ruptured to me.
 
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