68 Impala Disc Brakes - Impala Tech
Brakes & Suspension Conversion Questions & more

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post #1 of 12 (permalink) Old 07-24-2017, 06:42 PM Thread Starter
 
 
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68 Impala Disc Brakes

I have a 68 Impala with CPP power disc brakes in the front and the rear with a 7in or 8in booster and a vacuum canister. The problem i'm having is that I have a hard pedal. The brakes work, but not like a disc brake car should. Do anyone have a solution to the problems that I am having?
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post #2 of 12 (permalink) Old 07-24-2017, 09:11 PM
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Your car left the factory with an eleven inch diaphragm power brake booster. A seven inch is only going to yield less than 28.9% of the effort it takes to apply the brakes with an eleven inch diaphragm (area of a circle is ∏ r²). The smaller diaphragm installed so you can run tall valve covers as a fashion statement on a BBC is going make you apply four times the pedal effort as the stock part; but it looks cool doing it.

I make that statement as the only reason you run a tall valve cover is to clear the STUD GIRDLE bolted to the extra long poly lock nuts holding your roller rockers on top of your tenth of an inch taller valve springs to get enough open pressure to keep your roller lifter tappets in contact with the cam lobes. If you don't have these old school hard core race parts under your tall valve covers then the stock stamped stamped steel valve covers will work just fine and they allow you to use a bigger diameter power booster diaphragm.

Even a dual booster seven inch power assist system only offers you 57.8% of the pedal effort; of the stock eleven inch booster. Though you could go with a Hydra-boost brake system off of a diesel pickup truck if you do have Jesel shaft mounted rockers under your tall valve covers that are needed to resist flexing due an insane solid roller cam that produces only four inches of manifold vacuum at idle; preventing the idea of a vacuum power booster working at all.

Big Dave
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post #3 of 12 (permalink) Old 07-25-2017, 12:03 PM Thread Starter
 
 
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My car never had power disc brakes. I converted from drum brakes on all 4 corners to power disc brakes on all 4 corners to stop the 450hp that the 383 produce. The disc brake conversion kit came with the dual 7in booster I added the vacuum canister, but still a hard pedal.
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post #4 of 12 (permalink) Old 07-25-2017, 01:29 PM
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A vacuum canister works the same way a tank works on an air compressor; it holds the air out of the compressor. If you have no air pressure out of the compressor it won't hold much. Like wise the vacuum accumulator depends upon the lowest gauge reading of manifold pressure. If you don't have much manifold vacuum due to cam overlap keeping the valves hanging open it can not collect much vacuum to work with the brake booster.

You can add an electric vacuum pump off of a diesel engine powered truck (diesel engines have the manifold open to the atmosphere so they have no vacuum), or off of an early Cadillac or Buick that had everything on the car (windows, antenna, seat, etc.) powered by vacuum from the motor.

If you have a SBC, even with tall valve covers, an eleven inch power brake booster will fit. The larger diameter offers more surface area for the vacuum to work against. I am surprised they had you buy the much more expensive dual small boosters instead of the more common and much cheaper stock booster. Only reason I can think of is they wanted your money.

One other thing to look is a vacuum leak in regards to the check valve in the vacuum supply line. It seals the brake booster from the manifold when the engine is off so that you can stop the car when you first crank it up. There should be enough vacuum retained in the booster to allow the brakes to work.

Finally there are two holes in the brake pedal. One is for power brakes and the other is for manual brakes. You might try moving it to see if the change helps or hurts (I have no idea where it is set now).

I have to pull these suggestions out of the air because I am not there to see what is going on. A vacuum gauge reading from your engine idling would be helpful in diagnosing your problem.

Big Dave
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post #5 of 12 (permalink) Old 07-25-2017, 02:50 PM
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Check the booster and make sure it's working. Lately there has been a lot of complaints about CPP parts failing out of the box, especially boosters.
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post #6 of 12 (permalink) Old 07-25-2017, 09:29 PM Thread Starter
 
 
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When I pulled the check valve from the booster after the car has been running a few minutes you can hear the air coming out of it.
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post #7 of 12 (permalink) Old 07-25-2017, 09:46 PM Thread Starter
 
 
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Big Dave, I'll check to see which hole the brake pedal is in and check how much vacuum the motor is making tomorrow.
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post #8 of 12 (permalink) Old 07-26-2017, 10:56 AM
 
 
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Look into a hydraboost system. Electric vacuum pumps SUCK - very noisy.
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post #9 of 12 (permalink) Old 07-26-2017, 12:19 PM
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I would just use the stock 1968 Impala disc brake booster.

1967 Chevrolet Impala SS convertible - Ermine White C1
1967 Chevrolet Impala SS 396 - Marina Blue FF
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post #10 of 12 (permalink) Old 01-16-2018, 01:01 PM
 
 
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I'm looking at swapping to front discs. My plan was to keep the stock booster and swap the master cylinder to a disc/drum master cylinder. Having read the threads here and on other sites it seems all the conversions are using 11" discs deigned for the mid size cars. Is anyone aware of a kit that does 12" discs? I'm not interested in a 4 wheel disc set up at this point.
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post #11 of 12 (permalink) Old 01-16-2018, 01:55 PM
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Twelve inch discs were used on Police cars and Station Wagons (if you ever look in a parts catalog they all list these two vehicles as being excluded when describing the application). Today finding a sixties Police car in the junk yard is rare (because they are a source of a lot of heavy duty parts from electric to cooling as well as suspension and tranny), but there were a lot more station wagons made than police cars (9C1 SEO code). Additionally the same brakes were used on Pontiac Safari wagons, Olds Vista Cruiser wagons, and Cadillac limos.

Twelve inch disc brakes require a fifteen inch wheel (the 9C1 cars had a special HD steel wheel that was ventilated yet stronger than a stock steel wheel). Corvette rally wheels will fit even Girling fixed bridge four piston calipers, so they will work as well.

Big Dave
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post #12 of 12 (permalink) Old 01-18-2018, 12:05 PM
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Quote:
Originally Posted by wcoffey357 View Post
My car never had power disc brakes. I converted from drum brakes on all 4 corners to power disc brakes on all 4 corners to stop the 450hp that the 383 produce. The disc brake conversion kit came with the dual 7in booster I added the vacuum canister, but still a hard pedal.
Which brand kit did you go with?

Sent from my XT1254 using Tapatalk

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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