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Old 12-31-2007, 11:58 AM
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Brake Help!!!

Car: 85 Monte Carlo
9inch Ford rear end with aftermarket disc brakes
Problem: Since I have replaced the rearend and brakes on this car I have not been able to get the system bled. I have never heard of anything like this before. I have had someone pump them up and bled them. I have used a vacuum bleeder and still cant get them bled. When I changed the brakes out to discs in the rear I also changed out the brake valve for one that is disc/disc instead of disc/drum. Has anyone ever heard of this problem and how can I solve it? Any help would be greatly appreciated. I am ready to set fire to this thing!!

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Old 12-31-2007, 12:51 PM
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Have you bleed the master cly??? Some are a pain in the a--! Some of the tilted clyinders, we have had to raise the rear up about three feet. Crazy but true.
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Old 12-31-2007, 03:18 PM
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remember, not all master cylinders are compatable with a disc/disc setup. the oem master cylinder may not have the sufficient volume per stroke for a disc setup. you can call the experts at Master Power Brake, I've found them to be very helpful in the past. You can always use a corvette master cylinder like all the streetrod guys use with g.m. (metric) calipers and 9in. ford disc brake rearends, but I prefer to consult the experts in any brake problems I can't figure out. Just one word of advice from an 'ol fart-"don't cut cost on brakes,steering or suspension, you can fix a smoking motor or bad tranny, shabby bodywork,ect... you dont have to be able to go, but you have to be able to stop and steer!
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Old 01-01-2008, 09:36 AM
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make sure the bleeder screw is at the top of the piston.
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Old 01-01-2008, 08:10 PM
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I bled the m/c first. The bleeder screw is at the top of the caliper. I guess the next thing is try a different m/c.
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Old 01-01-2008, 08:45 PM
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a m/c for 4 wheel disc brakes must be used.
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Old 01-02-2008, 02:39 AM
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Disc brakes require more pressure to effect the same braking force as drums. Using a disc/drum master on a disc/disc setup will be all wrong.
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Old 01-02-2008, 09:01 PM
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Quote:
Originally Posted by Crosley
a m/c for 4 wheel disc brakes must be used.
be that as it may. the system should still be able to be blead out.
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Old 01-03-2008, 09:27 AM
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Quote:
Originally Posted by superhero
Car: 85 Monte Carlo
9inch Ford rear end with aftermarket disc brakes
Problem: Since I have replaced the rearend and brakes on this car I have not been able to get the system bled.
I am assuming you think the system is not bled because the pedal goes to the floor when stepped on.

I had the same experience and this is what I found.

The brake system was in fact bled fine.

What I found when bleeding my rear disc brakes (GM calipers with the built in emergency brake mechanism) is that the rear pads MUST first be adjusted to barely clear the rotor. A slight drag is about right. If this isn't done you will never get a "pedal" because the rear pads have to travel farther than the master cylinder is capable of performing. A second very important thing to consider is the pedal ratio. If you have non-power disc brakes the pedal ratio must be 6:1 or even 7:1 to work properly. For power assisted the pedal ratio works best at a pedal ratio of 4:1 or slightly less.

I found all this out primarily trial and error. I went thru three different Master cylinders (1" bore, 1 1/8" bore and a 7/8" bore). All were designed for 4 wheel disc brakes. I also am using a 7" dual diaphragm power booster. Originally the pedal ratio was set up at 6.5:1 which would have worked OK if I did not have the power assist. With the 1" or 1 1/8" bore master cylinders and that pedal ratio I had to "pump" the brake pedal twice to get the rear brakes to apply. Even then the pedal was "low".NOT GOOD!

I ended up with using the 1 1/8" bore master cylinder (this size flows more volume but delivers less pressure), the 7" dual diaphragm booster (increases the pressure) and a pedal ratio of 4:1.

Since my master cylinder is mounted under the floor I also had to install residual valves in the lines as close to the outlets on the master cylinder as possible. These will not be needed for a firewall mounted master cylinder. I also installed an adjustable pressure regulating valve in the rear brake line to enable balancing the system to prevent rear wheel lockup occurring before the front calipers were applied. You will need this valve.
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