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PostPosted: Wed Feb 17, 2010 1:47 pm 
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I have a 1970 Custlass with front power disc brakes. I know its been a while since 1970 but I seem to remember that power brakes on older cars were almost too easy to apply (brake pedal use). My car seems like it isn't getting much of any power assist and the prior owner has even installed a vacuum canister between the manifold vacuum port and the master cylinder booster. The engine is pulling 11 hg but I haven't tested the output from the canister yet. Is it supposed to be greater than manifold vacuum or does it just provide a reserve vacuum so there is always enough vacuum to the booster? So I guess I'm asking how a vacuum canister works and what to expect. The fact that there is a canister already in place when the manifold produces 11hg tells me something is not right. Could the booster be bad? Is there a way for me to test the booster?

My only prior experience was with another car with a big block and I installed a set of front Wilwood disc brakes on it. I had to use a smaller than normal booster behind the master cylinder because of space limitations and those were the most sensitive brakes I've ever seen. It would put you through the front window (exaggeration) with barely any pedal pressure and I was only getting 8hg due to a big cam.

Any thoughts on this would be much appreciated. Thank you.


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PostPosted: Thu Feb 18, 2010 1:23 pm 
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bobac455 wrote:
........ I seem to remember that power brakes on older cars were almost too easy to apply ........
All drum brakes were/are "touchy" when cold. After the first stop (or two) it goes away. The addition of power assist exaggerates the situation. Not so with discs as they do not work well until they have "warmed up". Power assist is helpful when they are cold.

Easier to understand when one realizes that drum brakes are far superior when cold, but lose efficiency as temperatures rise, while the opposite is true of discs.

Quote:
........ My car seems like it isn't getting much of any power assist ........
Seems? Compared to what?

Quote:
........ and the prior owner has even installed a vacuum canister between the manifold vacuum port and the master cylinder booster ........
Canisters are an aftermarket band-aid for use with a long duration cam and power brakes.

Quote:
........ The engine is pulling 11 hg ........

At idle? How much at a 30+ MPH cruise?

What cam?

Are initial timing, centrifugal advance, and idle speed, set to factory specs? Vacuum advance is not a factor at idle.

Quote:
........ Is it supposed to be greater than manifold vacuum ........
Not possible to draw more vacuum than is available at its source.

Quote:
........ does it just provide a reserve vacuum so there is always enough vacuum to the booster? ........
Not exactly. During periods of higher vacuum (cruising) the excess is stored for use when the brakes are applied.

I have 3 "A" and 1 "G" body with front discs. 2 ('70 & '87) with power and 2 ('50 and '65) without. Not much difference between the four.

Regards, Norm



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