Only enough for one good stop. 73 Duster

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Trevor B

Trevor B

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I seem to have a vacuum problem somewhere involving the brake booster.
We bled the brakes and they feel fine and stop the car fine. Actually, they work in general unless you pump them hard and then try it again immediately afterward. Not that there's much ice around Northern California but right is right and these brakes aren't.

I changed the check valve and the hose is good.

There do seem to be a few drops of fluid leaking from the front of the master cylinder when the engine has just been shut off. Certainly not much, though.

Any ideas?

Thanks!
 
That sounds like the valve in the center of the brake booster is sticking. When this happens the booster diaphragm dose not return back because vacuum is still being applied after you release the pedal. Since this valve is internal to the booster and not serviceable, replacing the booster is the only option if this is your problem. But first check the vacuum port, hose and check valve for any restrictions. One way to do this is to remove the vacuum hose from the intake manifold and the brake booster. Using compressed air blow it through from the booster end of the hose.. It should blow freely. And should not blow through at all the other way.
 
Either that oe the piston in the booster has crud/gum around its seal; you are moving the piston further and into a cruddy area when you push hard and it is sticking.
 
Okay - I checked the vacuum hose and check valve and they are fine.
I think perhaps it would be best to replace the whole business.

Any recommendations for a good place to get a rebuilt brake booster+master cylinder?
 
What size cam is in it?
 
The engine is bone stock.
Only modifications include removal of the EGR and plugging of associated ports on carburetor. But this problem happened before I did that and after as well.
 
You might also look at the power brake booster check valve.
 
Okay so it's been a while but today we put on a new master cylinder and booster.
Two things I've noticed:

1) Same problem as before: I can only get the brakes to really lock up if I gradually and slowly push the pedal until it's part way down, then step on it hard. STILL ONLY ONE GOOD STOP! Very frustrating (and not cheap... looks like I didn't need to replace the booster, at least)

2) Pulled off the back wheels and, although the parking brake is engaging the rear brakes, the pedal does NOT. We bled the rear lines again and there is fluid squirting out. Just no action.

Does anyone have any ideas about this? Anyone experienced this?

Please help!
 
Trevor B said:
2) Pulled off the back wheels and, although the parking brake is engaging the rear brakes, the pedal does NOT. We bled the rear lines again and there is fluid squirting out. Just no action.

Does anyone have any ideas about this? Anyone experienced this?

Please help!
Click to expand...

Did you, or have you replaced the wheel cylinders? If the car has been sitting, they could be corroded and frozen in the bore.
 
Have you ever put a vacuum gauge on the engine?
 
Bruce: Yeah, it had been sitting for 10 years and we thought that would be worth exploring. I actually just ordered a pair of cylinders and we'll try that next.

RustyRatRod: Still really can't make heads or tails of the vacuum thing. I have not put a vacuum gauge on the engine. What might I be looking for, numbers-wise?
 
Stock engine should be 20 inches of vacuum if it is in good shape. My stock 340 only had 15" of vacuum but it need some work. The bigger the cam is the lower the vacuum will be at idle.
 
Well... it's showing 13 - 14" on the vacuum gauge. Not very encouraging.
Is this too weak to make the brake booster work properly?

EDIT: We put in the new wheel cylinders today. Old ones were, indeed, seized up.
Now getting some more braking power but this vacuum thing is really baffling.

The brakes work and feel great if you push down the pedal gradually. The car stops the way it should stop.

I cannot, on the other hand, slam my foot onto the brakes and instantaneously get the benefit of the booster.
 
Trevor,

If you do a search on how power brakes work, you will see that all the booster does is assist. Which is why when you press slowly on the brakes all feels fine, but I suspect you are feeling what we all do with manual brakes. You should also know that the pedal to piston travel is generally less on manual brakes than on power assist brakes. This gives a greater mechanical advantage but longer throw with manual brakes. So required pedal pressure is less to achieve same stopping power. In your case you are likely not getting much assist because your vacuum may be too low. Generally you need at least 16 inches of vacuum at idle to generate assist. So your pedal to piston travel is likely less than a manual brake car, which means without assist it is harder to stop the car. You can check your brake pedal to see if there is a hole further from the pedal to connect the brake booster too and see how that effects the effort on braking. Keep in mind that the futher from the pedal you connect the booster, the longer your pedal travel will be. You can also try adding a vacuum canister. About $60 at Summit. Your engine is close on needed vaccum and the canister will see higher vacuum on deceleration as an example. The idea hear is that the canister accumulates vaccuum and holds it. Then when you hit the brakes there is sufficient vacuum to provide assist on repeat applicatons of brakes. Of course there is a limit and likely in heavy stop and go driving the benefit of the canister will drop off rapidly. This is where using an electric vacuum pump can help.

You also need to find out why your vacuum is as low as it is. If engine is stock it should be higher. You may have a vacuum leak. How does the engine idle? If it is rough and you hear some popping sounds at the exhaust, you have some engine issues.

I am considering going to vaccum assist brakes, but becasue of the cam in my 340 I will likely need a pump and canister. I am around 11 inches at idle. It is tough woeing this puppy down with manual disk/drum brakes.
 
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