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  #1  
Old 10-23-2017, 03:00 PM
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sidewalkman sidewalkman is offline
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Default Brake MC question

Probably a stupid question, but I'm asking it anyway.

I'm going to hopefully over the winter upgrade to a new double master and 8" Vacuum Boost but I see there are 2 flavours (Canadian for flavors), Disk / Drum and Disk / Disk.

I'm leaning toward all 4 wheels being disk since the costs to do it myself with all the tools and tips I've gathered here make it not too more to do the rears as well as the front.

BUT that's not this winter but sometime in the coming year or 3. I'm wondering if the Disk / Disk would work for my stock set up of drum / drum till I go all disk?
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  #2  
Old 10-23-2017, 04:40 PM
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Default

Lots of variables but it should - don't think I would risk it though.

The MG's I work on have the same style system as the Tbird. 4 wheel drum back to a single line master cyl. When MG switched to front disc they only added an extension to the master cyl to give more fluid capacity - no other brake changes.

From this

To this


A few variables:
After the MG went to discs it was still a single line system. When you go to 4 wheel discs on the Tbird I'm guessing you will split your front and rear systems add a proportioning valve or combo valve - no longer a single line system. Prop valve would most likely have the wrong pressures so no telling how the front to back balance would be.
You'll have to change your brake light switch - either go to disc pressure switch or convert to electric.

Maybe some others will have ideas on it.

Eric
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  #3  
Old 10-23-2017, 05:43 PM
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Most of us who retrofit bought either a Mustang-type or a Corvette-type master cylinder with a one-inch bore. Both work equally as well and either can be used for disk/disk OR disk/drum. These master cylinders are a straight shot with no residual valve or any restriction. They are simply two pistons on the same shaft (or spool). So, both pistons output identical volume.

Since disk/drum uses different pressures for front and rear circuits, you must match the correct combination proportioning valve (which is where lower pressures are created for the rear circuit). Disk/disk systems also use a combination proportioning valve but pressures on both sides are the same. Let me make this clear. Different pressures are not a product of the M/C. Different pressures come from the combination proportioning valve.

Front and rear pistons are different size diameters so there is a difference in the amount of fluid required. This is VOLUME. Since there is a difference in your brake volume between front/rear circuits, the prop valve ties both circuits together with a mechanical spool. This applies to either disk/disk or disk/drum.

Pressure is always created by the resistance to flow, when the pads/shoes contact metal and stop motion.

I hope you are considering a TWO-STAGE 8" booster. It is about the same diameter as an OEM booster, so it fits a Squarebird. The difference is, disk brakes require a lot more boost. The two-stage 8" does the trick nicely but if there was more room I would do a larger booster. I'm still happy with my two-stage 8" booster. - Dave
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  #4  
Old 10-24-2017, 01:39 AM
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Quote:
Originally Posted by simplyconnected View Post
Most of us who retrofit bought either a Mustang-type or a Corvette-type master cylinder with a one-inch bore. Both work equally as well and either can be used for disk/disk OR disk/drum. These master cylinders are a straight shot with no residual valve or any restriction. They are simply two pistons on the same shaft (or spool). So, both pistons output identical volume.

Since disk/drum uses different pressures for front and rear circuits, you must match the correct combination proportioning valve (which is where lower pressures are created for the rear circuit). Disk/disk systems also use a combination proportioning valve but pressures on both sides are the same. Let me make this clear. Different pressures are not a product of the M/C. Different pressures come from the combination proportioning valve.

Front and rear pistons are different size diameters so there is a difference in the amount of fluid required. This is VOLUME. Since there is a difference in your brake volume between front/rear circuits, the prop valve ties both circuits together with a mechanical spool. This applies to either disk/disk or disk/drum.

Pressure is always created by the resistance to flow, when the pads/shoes contact metal and stop motion.

I hope you are considering a TWO-STAGE 8" booster. It is about the same diameter as an OEM booster, so it fits a Squarebird. The difference is, disk brakes require a lot more boost. The two-stage 8" does the trick nicely but if there was more room I would do a larger booster. I'm still happy with my two-stage 8" booster. - Dave
So either way I'm looking at a prop valve. I was thinking I'd need it in any case since the front brakes always do the majority of braking since the weight transfer is to the front.

I'll also look into a 2 stage booster. I didn't realize there was such a thing.
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  #5  
Old 10-24-2017, 03:11 AM
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They're actually called, 'dual diaphragm' boosters.

The booster has a 'step' in the case that is easy to spot. CLICK HERE for an example.
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