no power brakes on idle, great at speed? - Pontiac GTO Forum
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post #1 of 21 (permalink) Old 04-11-2016, 11:14 AM Thread Starter
 
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no power brakes on idle, great at speed?

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

Last edited by busaben; 04-11-2016 at 01:44 PM. Reason: clarification
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post #2 of 21 (permalink) Old 04-11-2016, 01:17 PM
 
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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

"Honest Honey, it just kinda followed me home, can I keep it?"
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post #3 of 21 (permalink) Old 04-11-2016, 01:34 PM Thread Starter
 
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glad that link helped you out, not so much for me
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post #4 of 21 (permalink) Old 04-11-2016, 04:09 PM
 
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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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post #5 of 21 (permalink) Old 04-11-2016, 07:10 PM
 
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Factory booster or aftermarket?
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post #6 of 21 (permalink) Old 04-11-2016, 09:59 PM Thread Starter
 
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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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post #7 of 21 (permalink) Old 04-12-2016, 08:36 AM
 
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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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post #8 of 21 (permalink) Old 04-12-2016, 10:43 AM
 
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When is the last time you replaced you vacuum hoses. Fix the simple stuff first.
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post #9 of 21 (permalink) Old 04-12-2016, 11:16 AM Thread Starter
 
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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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post #10 of 21 (permalink) Old 04-12-2016, 06:17 PM
 
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Originally Posted by busaben View Post
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?
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