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Old 03-11-2005, 02:11 AM
Joey Buttafucco's Avatar
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help with my pooey brakes

hello peoples...just have a few questions

i have a 64 Impala Wagon that i have already retro fitted the front brakes w/ 1979 corvette spindle and respective 12 inch disks and 4-piston calipers. very cool that these just bolted on..

now i have to deal w/ the rear breaks which are stock drums. i do have a stock rear-end and the current stopping power is crappy!!! i have a master cylinder from a 69 camaro that was originally mated with the 4-piston calipers and rear drums. i cant stop worth beans! my stock 4-wheel drums worked better it seems.

now are there any rear disk applications for my ride that are from some other car (i would prefer if there is an "almost bolt-on" application but if mods must be made, i will be able to do so...NEEDING YOUR HELP OF COURSE)...

i am currently doing the entire rear suspension and putting hotchkis everything so i hope to attain a measure of performance for my dear old lead-sled. then i will be putting the full AIRRIDE SHOCKWAVE kit.


Joey BUttafucco OVAHERE

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Old 03-11-2005, 06:22 PM
abstruse dutch uncle
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There are several rear disc kits which could work, but if they stop crappy now, that isn't necessarily the fix for it. Drums actually have an advantage over discs in that they work at a lower line pressure, so if your pressure is low now, discs wont help. The advantage discs have over drums is mostly in heat dissipation. If they stop crappy cold, discs wont help... if someone claims they're car stops quicker with discs, even when cold, there is usually another explanation... once the tires can lock, you cant make them 'lock more'.

If your pedal is high and hard, you may have a pressure problem (not enough). If it is power assist, look at the booster, pushrods, and master cyl. If it is manual brakes, could be too large of a master cyl. Should be a 1" for those brakes, or even a 15/16, depending on your pedal ratio. You might even want a 1" with power brakes if you are running a 10" or smaller booster, or the pedal ratio is less than 3.5:1.

If your pedal is low and mushy, look at pushrod length/adjustment, look for air in lines, shoe adjustment, or leaks. Corvette calipers have spring loaded pistons, so if the disc is not held straight during work, or if the disc has excessive runout, the pistons will 'cant' and let air in around the seals.

Also, the '69 Camaro master cylinder is definitely a 1.125". The disc/drum master was designed for a caliper with a larger displacement than the 'vette's, so might cause a hard pedal. If it is an aftermarket master, it is probably missing the residual valve to the rear drums too, which might allow a mushy pedal.
The replacement for the Corvette and the '69 Camaro are often listed as the same part, but the Corvette used a dual diaphragm booster with power brakes, and a 1" master with manual. I beleive the 4wl disc (JL8) Camaro may also have had a dual diaphragm booster, but not sure. Also not sure if it came with manual brakes with the JL8 option... I know the regular 69 disc/drum, single piston, were power only. The 67-68 disc/drum cars had a 1" manual master.

Last edited by yesgo; 03-11-2005 at 06:31 PM.
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Old 03-23-2005, 01:45 AM
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bad braking...

i dont want to try and hijack a thread here so ill try and make it sweet... i have the same problem as joey here, except i have front disc stock... and drummed rear.. ive replaced everything but the lines... (had them flushed).. i dont have any stopping power.. im really low on cash now as well or i would go with a whole new set up... now i have a seriously hopped up motor... cammed out and all that jazz ... now will that effect my pressure to the booster... by the way im runnin with a 71 gto.. 455ho raIV .. now if its a vaccum problem because of the cam... (kinda what i was thinkin.. but dont know) is there an aftermarket booster kit. or vaccum booster.. that would improve my breaking... thanks anyone that can help... and sorry about 1/2 jackin the thread.... wont happen again... just kinda hit home here....
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Old 03-23-2005, 02:33 PM
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my guess

My guess would be the proportioning valve. They are factory set for certain combinations. You change the combination the proportioning valve doesn't work properly. You can buy a manually adjusted proportioning valve. Jegs adjustable proportioning valve. As far as changing to 4 plunger caliper. Chances are you master cylinder cannot deliver enough volume of fluid to fill 10 plungers in one pump. Purchase a Corvette MC.
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Old 03-23-2005, 10:09 PM
abstruse dutch uncle
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Yes the proportioning valve can effect the amount of pressure that gets thru to the rear brakes, if you have one installed. It could cause a hard pedal if it is defective. The wrong valve could cause under/over effective rear brakes during hard braking, but should not effect normal braking.

A 1971 GTO with power disc brakes should have a combination valve, so make sure you are getting fluid thru to all 4 wheels. It has a metering device which delays flow to the fronts, as well as a proportioning valve to reduce line pressure to the rears. If either is stuck closed, it could cause a hard pedal.

On the Corvette master, most parts suppliers will give you the exact same part for a 67-76 Corvette master cylinder as a 1969 Camaro master cylinder. Either one stock had enough volume in the front circuit for the Corvette calipers or the Camaro calipers... or the GTo calipers...., but the current design is for a disc/disc Corvette, which sends more volume to the rear brakes and has no residual valve, so I would avoid it on a disc/drum system. I have no further comment for Joey as i dont know if he had a hard or soft pedal, and hopefully has found the problem by now anyway.

For 71_gto, the vacuum assist would also be my guess since you said you are running a big cam and have a hard pedal. You should still make sure fluid is getting thru, but you can test your engine vacuum. You should have at least 14" to run that booster. More would be better. If you have less than 14" of vacuum, you can buy a reserve canister, or a pump. I believe summit or jegs has either or both.

Test your vacuum and test the vacuum check valve to the booster. Also test the booster with a hand pump.
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