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Old 11-23-2010, 06:48 AM
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step bore master cylinder

I have 4 wheel disc on a 1934 ford. Under floor 1 1/8" bore master cyl.with 7" boost ,chevy 11" with metric calipers on four corners. With a very low peddle. Been thru the bench bleed, pressure bleed four corners, install correct with bleeder at top, plumbed correct with gm combination valve, and 2# residuals frt and rear. In reading have seen several suggestion on running a step bore master cylinder. Question is: has anyone used one in a underfloor m/c metric disc application and if so witch m/c.

Thanks guys

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Old 11-23-2010, 07:47 PM
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come on guys, somebody has to know which one
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Old 11-23-2010, 08:01 PM
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I also have a 34 Ford with 4 wheel discs, but a manual master cylinder. I have zero problems with stopping ability. My MC is a 67 Corvette 4 wheel disc manual.

Vince
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Old 11-23-2010, 08:26 PM
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Is the master cylinder below floor? Or is it on the firewall.
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Old 11-23-2010, 08:28 PM
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oh and what calipers are you using? by the way thanks for replying 302 z28
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Old 11-25-2010, 12:53 AM
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IF (big IF) you have low-drag calipers (low drag for improved fuel economy) you must have a step-bore (quick take-up) master cylinder.

If you don't have the low-drag calipers, a QTU master is just crutching a defect you haven't found yet.

I suspect trapped air in the master cylinder. Is the front of the master tipped "up"? Or is the master level or tipped down in front?

Residual pressure valves with disc brakes??? That's somewhat scary.
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Old 11-25-2010, 05:55 AM
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Thanks Schurkey for reply. First I still have the Corvette 1 1/8" bore master cyl and it is level under the floor. Guess it could be air. That is exactly what it feels like (spongy). But I have pressure bled over a quart of fluid thru it. Residuals? I have the brakes plumbed just like Master Power,Speedway,CCP, and ECI shows. Do you know which step M/C to use? GM has more than one.

Thanks
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Old 11-25-2010, 06:35 AM
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sorry not CCP CPP
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Old 11-25-2010, 06:53 AM
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Check your pedal ratio.

Most aftermarket under the floor pedals are set up for manual brakes and have a 6:1 pedal ratio.

When using the power brake setup and the 1 1/8" bore master, the pedal ratio must be in the 4:1 area to maintain a higher pedal. The pedal should be fairly high and hard until you start the engine to get vacuum to the booster. At that point with your foot applying some light pressure to the pedal, the pedal should drop about 1/2-1". Any additional foot pressure will apply the brakes.

Another point of interest. IF you are using the GM rear calipers with the built in cam actuated parking brake; you must run a 10 psi residual in the rear lines. This is because these particular calipers have up to .050 of piston travel while the front GM calipers only have about .015. Using a 10 psi residual in the rear will keep the pistons from retracting the full .050. The calipers MUST be adjusted properly and the parking brake must be used as this is what keeps the rear calipers properly adjusted. If you are running these rear calipers and they are not adjusted correctly the pedal will always be low and the rear brakes will not be fully actuated when the brake pedal is applied. Easy to test. Jack the rear wheels off the ground. Have someone apply the brakes and hold. See if you can turn the rear wheels. If you can then the rear calipers are not adjusted correctly.
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Old 11-25-2010, 07:42 AM
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Quote:
Originally Posted by maninacoupe
Is the master cylinder below floor? Or is it on the firewall.
Below floor.

Vince
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Old 11-25-2010, 07:44 AM
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oh and what calipers are you using? by the way thanks for replying 302 z28
Willwood 4 piston forged polished.

Vince
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Old 11-27-2010, 09:54 AM
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302 z28,
Thanks for the info. Is that a Street Beast body kit
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Old 11-29-2010, 12:28 PM
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thanks to all you guys. Got full full peddle and brakes. After finding out that in all my trying different m/c's. I had picked up the wrong m/c and put it on the car. Had a 7/8" bore m/c on the car. I put the right one on(1 1/8 bore). Got brakes and good peddle. Sorry for wasting you you all's time, but again thanks for input. Have learned a lot of valuable info in this episode.
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Old 09-02-2011, 10:20 PM
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what 1 1/8" master cylinder are you using? I am having the same problem you first described but with disk/drum. With my 7" booster, I have not found a step bore mc that will fit. Currently have a 1" 79-81 buick Riviera mc.

With back brake port blocked, I can get the brake pedal almost all the way to the floor so it must be the front brakes. Thinking I have no drag calipers.
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