# no power brakes on idle, great at speed?



## busaben (Oct 30, 2008)

its been a long time, but im tinkering with my car again, let me describe what the issue is. i replaced all my drum brakes with the 4 wheel disc brake conversion from summit, and ran all new lines. at speed the brakes work great, good solid feel and slows the car down in a hurry, but at a traffic light i have to stand on the pedal to keep it from inching forward, it has firm pressure, but not acting like the booster is working.

i put the car in my garrage for the winter/work outages. months later i resumed troubleshooting the brake issue. started by inspecting the check valve and herd the hiss of vacuum as i removed the valve from the booster. now if the booster can hold vacuum for a few months, then the check valve and booster should be good right?

now if the engine is making vacuum, check valve and booster holding vacuum, then how is my brakes not working on idle? i checked the vacuum on idle a while ago and it was 15-17"Hg. my Question is can the booster still be hosed up in such a way that it doesnt loose vacuum, but doesent power the brakes? any help would be great


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## Red1970GTO (Jun 25, 2011)

There are lot's of questions about your system/install that you haven't answered... Check this out: How to Troubleshoot Brake Problems - For Dummies


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## busaben (Oct 30, 2008)

glad that link helped you out, not so much for me


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## Lemans guy (Oct 14, 2014)

I would rebelled your hydraulic system, those calipers can trap air in the lines and make the pedal feel weird because the air does not compress. Make sure those caliper bleeders are at the very top of the caliper when you bleed it, sometimes I have taken the caliper off the rotor to orientate it so it was getting all the air, a power bleeder is best though, if you don't have one and can drive it to a shop, just bleeding it can be not too expensive.

I have been absolutely sure I had all the air out before, except I didn't...I would try that first and then if that does not suffice, start down the tree of rechecking for sticking calipers, booster somehow sticking, or master cylinder plunger sticking....

But re-bleed first....you will get it!


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## PontiacJim (Dec 29, 2012)

Factory booster or aftermarket?


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## busaben (Oct 30, 2008)

Power bleed the brakes again, all bleed screws are on top as they should be, calipers are not stuck or sticking.

It has an aftermarket(summit racing) 9" booster. I'm thinking I either don't make enough vacuum at idle to sufficiently power the brakes, or I need a larger booster for more vacuum storage.


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## Lemans guy (Oct 14, 2014)

you may be right about booster size, I think there is 11 inch as well. Anyway, try putting a vacumn gauge between the check valve and the booster, just splice a hose in and splice the gauge in and then let her idle and see what the vacumn actually is inside the booster at idle.....

as opposed to what manifold is pulling, ..........when driving those high RPM's at light throttle will produce good vacumn. also make sure that your booster line is hooked up to manifold and not ported vacumn...apologize if you already checked that, but ported vacumn at idle would produce no or very little vacumn and at light throttle cruise would produce more......

the connections at the back of carb sometimes are confusing on where that vacumn is coming from...........but the smaller booster may use up the vacumn faster and not give you enough stored vacumn, it has to constantly be replenished, ......


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## 68GTO4004Spd (Jun 19, 2008)

When is the last time you replaced you vacuum hoses. Fix the simple stuff first.


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## busaben (Oct 30, 2008)

All hoses were replaced last year, while I was finishing up the restoration, it's basically a new car at this point, that's why I'm leaning twards a component that just don't match my setup. 

How much difference would an 11" booster make over a 9" booster? I'm asking because I don't know, but at normal driving speed, at its peak vacuum it brakes great, better than my late model cars. It's at the low speeds/ lower idle that the brakes suck


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## PontiacJim (Dec 29, 2012)

busaben said:


> All hoses were replaced last year, while I was finishing up the restoration, it's basically a new car at this point, that's why I'm leaning twards a component that just don't match my setup.
> 
> How much difference would an 11" booster make over a 9" booster? I'm asking because I don't know, but at normal driving speed, at its peak vacuum it brakes great, better than my late model cars. It's at the low speeds/ lower idle that the brakes suck


Not an expert, so just throwing this at you, but if you do some researching, the 9" booster sounds like your problem _UNLESS_ you have what they call a "dual diaphragm" 9" booster. The single 9" will not give you the same pressure as the factory 11" and that could be where your problem lies. Seems the dual diaphragm in many of the blogs is what is used to replace the factory 11" for clearance issues. 

Your vacuum may also be a tab bit low and some use a vacuum can to enhance this like this Comp Cams can: COMP Cams Vacuum Canisters 5200 - Free Shipping on Orders Over $99 at Summit Racing 

Finally, the bore size of your master cylinder is important, but I assume your conversion came with a matching master cylinder? :thumbsup:


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## busaben (Oct 30, 2008)

I don't believe it's a dual diagram booster, it is not near as deep as the ones I have seen.

If the volume of vacuum is the issue I can add a canister to make up for the smaller booster that I have, however if low vacuum is the problem then changing to a larger booster or adding a canister won't help. 

Looks like it's about time to start slinging parts,so starting with the cheapest I'm going with a spare canister that I have laying around, if I can find where I put it.

And yup, the kit came with the master cylinder.


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## Lemans guy (Oct 14, 2014)

Is the brake booster the only thing on that Vacumn line? Could something else be coming off it? 

All the blend door controls are Vacumn activated.....but if you have full "manifold" Vacumn to the booster with nothing else branching off, 

Then

...it may be the wrong size booster as discussed


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## busaben (Oct 30, 2008)

i installed a vintage air system, so no vacuum doors. about the only things using a vacuum are the distributor advance, and the booster


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## Rukee (Feb 8, 2007)

Don't you figure circles by Pi? If so an 11 booster would give you just under 6.3" of additional surface area for the vacuum to pull on. But like mentioned above, check how much vacuum you have between the valve and the booster at idle. Should be the same as manifold vacuum, and be sure you have the right size fitting on the manifold.


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## busaben (Oct 30, 2008)

Cold engine (choke) vacuum was 17-18", hot engine idle was 16". Chopped the throttle a few times like normal driving and hit 20" regular. It appears that my vacuum might not be my issue, but the small booster size is. I'm getting my booster vacuum at the back of my carb (Holley 670 street avenger)


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## PontiacJim (Dec 29, 2012)

busaben said:


> Cold engine (choke) vacuum was 17-18", hot engine idle was 16". Chopped the throttle a few times like normal driving and hit 20" regular. It appears that my vacuum might not be my issue, but the small booster size is. I'm getting my booster vacuum at the back of my carb (Holley 670 street avenger)



Vacuum doesn't seem bad at all. Did you happen to check your vacuum as you were applying the brakes to produce the braking issue you are having? That might reveal something IF there is a vacuum problem. 

Here is one more thought. If the brakes work well at high speeds and on down, but not so good at low speeds just cruisin', I am wondering if the brake pads you have are metallic/too hard. Metallic pads work best as they heat up. Not so good if they don't. I don't use metallics for this reason and go with the most inexpensive pads I can buy as they seem to work best for the average everyday driving AND won't wear out the discs anywhere near as fast as metallics do. That said, my next experiment might be to get those brakes/pads hot from doing a few good high speed-to-stop runs and once you felt they were warmed up, then immediately follow with some moderate braking at lower sppeds to see if you notice any better braking results. Might not do a darn thing, but again, one more thing eliminated. 

If the braking test did not show any change, at this point I would spring for a dual diaphragm 9" booster just to try it (unless you want to go back to the 11"). They don't seem to pricey and if you do a web search you may find you can get one to fit your application through your local auto parts store.

If that does not work, I don't know what else you could check short of purchasing a line pressure gauge that will tell you line pressure going to your brakes. Here is one: Brake Pressure Gauge Kit - Sure Stop - A1704

Ya just gotta love it when this stuff is just supposed to bolt in and work and doesn't. :yesnod:


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## busaben (Oct 30, 2008)

I didn't check it while applying brakes, will do that next day or two, I have my booster vacuum coming off the back of my carb, would it be better to pull it off the manifold? Noticed there was a port that I could use if it was any benefit. Next step will be pads, booster, or vacuum cannister


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## Lemans guy (Oct 14, 2014)

So their are two vac sources on these cars....and you cannot tell what they are by looking or where they come from . You have to know or check with a gauge..

One is ported or above the throttle plate internally, and full manifold is below the plate, can come from carb or off direct manifold...

If by chance Ben you were hooked up to PORTED vacuum....it would produce little or no vac at idle and more under light throttle cruise....it would deliver what you are experiencing...vac accumulates in booster from light throttle cruise....no vac at idle.

The booster must be hooked to full manifold, and ideally alone on the line so no other component, even a vac advance in distributor can leak and reduce vacuum .

So first be sure you have full manifold vac to booster, check with gauge, and make sure...if it was ported you found the problem..if not make sure there was no leak in any component sharing the lime. The vac can in the dizzy has a chamber to hold vac and the can does change at idle at speed...so make sure that is good, take it out of the vac circuit and test or change for a know. Good one. They cost $10, or so if you don't have a new one.

If that all checks you are back to booster and other things discusssed


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## BearGFR (Aug 25, 2008)

Good catch, LeMans Guy. If he's running off a ported vacuum source that would certainly explain it.

Bear


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## Lemans guy (Oct 14, 2014)

Yeah Bear, Ben is on it.....let's hope it is something that easy.....if not he may have to regroup.....on that booster....make sure it does not leak internally or is too small or something.....

Anyway, let us know Ben


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## busaben (Oct 30, 2008)

the vacuum numbers i got were on the large vacuum port on the back of the carb, which seemed right with full manifold vacuum. the intake manifold also has a plugged port that i could use if there was any doubt or benifet. i checked the vacuum going to the distributor and it was ported as it should be, plugged all vacuum lines (dizzy, and pvc are all other than the booster)

ordered a canister today to see if it gets better, with 16-20" of vacuum on idle it should do better than it does


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