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Old 03-27-2011, 10:15 AM
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Pedal Ratio & MC

I have a 1955 GMC truck. Power brakes with 7" dual MC. The MC is 1" bore corvette style mounted on frame. I have a 1988 corvette front suspension and a 1980 camaro rear end with drums. {disc/drum}
The problem I was having is the rear driver side was dragging an getting hot. I replaced rear brake shoes and wheel cylinder and the rubber hoes on the rear end. It still did the same thing. I then adjusted the brake leakage that hooks to the pedal to booster. That stop the rear brakes from dragging, but my pedal is about 3" from the floor when I press on the brakes. I think the bore size is to big not getting enough pressure to the front caliper. I have been reading about pedal ratio to MC. My pedal ratio is off making the brake not work like they should. My pedal ratio I think is 2:1 . I have 12 inch from the pivot point to the center of the brake pedal and 6 inch from the pivot point to the Booster.
Any advice would be great. A fire wall mount mite work better?

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Old 03-29-2011, 08:04 AM
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brakes

You say you have power brakes, where is the booster mounted? You did not say if you have a proportional valve in line. The proportional valve is what controls the amount of pressure to the back brakes to keep from locking up. If you have a manual disc brake setup the 1" master is the right size. Most of the time if you have power disc brakes you would use a 1 1/8 master. Your pedal should be adjusted to where braking begins when the pedal is moved in about 1/4 inch.
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Old 03-29-2011, 08:14 AM
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http://www.autoanything.com/brakes/65A3153A0A0.aspx



Cole
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Old 03-29-2011, 12:09 PM
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A 2:1 pedal ratio would be a killer! For instance, the pedal ratio for a C3 is 5.40:1 for manual and 3.38:1 for power brakes.

Below is the formula in a diagram to give the pedal ratio, from a very good paper on brakes HERE:

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Old 03-29-2011, 12:10 PM
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pedal & mc

Everything is mounted on the frame. The booster proportional valve & MC. I have a 10lb residual valve for the rear drums. I order a 8 in booster. I hope it helps. I don't have room for anything bigger.
Thanks
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Old 03-29-2011, 01:53 PM
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re-read the reply from Cobalt327, your problem is not booster size it is pedal ratio. You need at least 4-1 for power brakes and 6-1 for manual brakes, with a 2-1 ratio you will need the leg muscles of an olympic weight lifter to stop your truck! Increasing the pedal ratio will increase your brake pedal travel so you may have to re-engineer some mounts but until you fix the pedal ratio you will never have acceptable brakes.
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Old 03-30-2011, 09:13 PM
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pedal & mc

Thanks for everyones help. I have brakes now. I put a 8 in Booster. My pedal ratio is 6:1 I measured it the wrong. Its not 4-1 but its better than 2:1 The good thing is my brakes now.
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Old 03-30-2011, 09:16 PM
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Good deal. Any chance the brake pedal arm has different mounting holes to change the ratio?
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Old 03-31-2011, 03:24 AM
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pedal & mc

No it has the stock pedal only one hole. It's 100% better. My legs are pretty strong. lol
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