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Discussion Starter #1
I'm so close to getting my manta on the road. But I just discovered that the booster is probably bad. Evidence of this is, when you start the car, the brake pedal pulls itself to the floor. And when you accelerate, it comes back up.

I've fixed all the other vacuum leaks which leaves one left... the booster itself.

If anyone one has a working one they'd like to sell, please let me know!
 

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Opel Rallier since 1977
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Mmmm... it actually sounds like your issue is in the master cylinder (and possibly the wrong one) or another part of the brake system. It also sounds like the inline check valve between the engine and the booster is not there, or is not operating properly; there should be no upward movement or pressure on the pedal when yo accelerater. The check valve keeps the vacuum inside the booster steady even when the vacuum in the intake manifold drops under acceleration. This check valve is about 2" long with the nipples over which the booster vacuum hose slips.

Here are a couple of test to run; let us know what you find. With the engine not running, you ought to be able to press the pedal and get a hard pedal after 1-2" of movement with no need to pump it. If you then hold that, and start the engine, the pedal then should sink a bit more to the floor.
 

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Discussion Starter #3 (Edited)
Mmmm... it actually sounds like your issue is in the master cylinder (and possibly the wrong one) or another part of the brake system. It also sounds like the inline check valve between the engine and the booster is not there, or is not operating properly; there should be no upward movement or pressure on the pedal when yo accelerater. The check valve keeps the vacuum inside the booster steady even when the vacuum in the intake manifold drops under acceleration. This check valve is about 2" long with the nipples over which the booster vacuum hose slips.

Here are a couple of test to run; let us know what you find. With the engine not running, you ought to be able to press the pedal and get a hard pedal after 1-2" of movement with no need to pump it. If you then hold that, and start the engine, the pedal then should sink a bit more to the floor.
The check valve is indeed there and verified to be working properly. Tastes like crap but works well.

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.
 

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Opel Rallier since 1977
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OK, sounds like you have done a fair amount of testing. I'm sincerely not trying to make things more difficult for you at all, but trying to help figure this out. The odd things are (if I read your symptom descriptions right):
  • The symptom of the pedal being sucked to the floor when you start and idle it. There is no way that I know of that this can happen with just the booster action, good or bad. That is why the questions on the MC being the correct one for the year and the rest of the system. The MC changed in '73 or '74 IIRC. With the engine stopped, where does the pedal go if you push on it very hard?
  • The changes in pedal position with high and low vacuum. The engine vacuum is obviously controlling the booster action and this simply should not happen with a proper check valve operation. Please don't take offense, but the check valve testing may not have been adequate. BTW, is the arrow on the check valve pointed towards the intake manifold?
  • The grease test may or may not be giving you the right hints. If you block the vent going from the passenger compartment into the booster, then the booster diaphragm cannot move and there is no effect seen on the engine operation.
BTW, in general, where are you located?
 

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Opel Rallier since 1977
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And BTW.. I can think of one way in which the pedal movement to the floor happens with engine changing vacuum levels: The control valve in the booster is stuck open (engages the booster all the time), and the check valve is not working right.

Do you have a pix of the booster installed? I have 3 and 2 and they look to be in good shape. My parts book says the Model 50 chassis from later in '72 through '74 take the same replacement part. But I cannot be sure these are before or after that change in later '72.

The '75 is different and larger per the Opel parts books.
 
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