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If ABS is involved, DOT 4.
All else gets DOT 5.

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Gregg55_99 asked: "Since this thread was brought back from the dead, a question. What master cylinder is used for this dual disk conversion?" Always use the Harley 11/16" master cylinders found on late model H-D Baggers [Road Kings, Electra Glides, etc.]. However Harley did make a 3/4" master cylinder and Brembo's got out a radial RCS19 radial style master cylinder with a 1" handlebar clamp for Harleys. Handlebar Master Cylinder 3/4 Bore Black fits Harley-Davidson | eBay
Brembo RCS19 GP Radial BRAKE w/ FOLDING LEVER for 1" bars (HD/V-Twin) (FREE EXPRESS SHIPPING ) 110.A897.10 (hardracing.com) The 11/16" master cylinder has a .687" bore diameter. The H-D 3/4" master cylinder is .750". The Brembo RCS19 has a 19 mm bore dia. or about .748". All thing being equal the bigger the bore the more fluid the piston can push resulting in more stopping power. These Brembo RCS19's are sold as replacement units for crotch rockets are are supposed to be the last word in motorcycle stopping power!
 

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Since this post was about H-D brake upgrades.I decided to post my custom front end I built for my '05 Dyna. It's got 08+ Dyna 49 mm forks with midglide triple trees, a Sportster XR1200 wheel, dual Tokico 6 pot 'Busa calipers and dual 320 mm Brake Tech rotors. I'm running a Road King 11/16" master cylinder. I'm running DOT5 in the brake system after thoroughly flushing out the calipers and installing new seals and pads. Unfortunately last June I ran into Bambi's sister on the road home and my front end was trashed! I'm going to rebuild with all the same components and probably upgrade to the Brembo RCS19 master cylinder.
Tire Wheel Automotive tire Bicycle tire Vehicle
 

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Just to clear up some of my confusion.

Was the 11/16" master cylinder ever used with the dual disks for the early FXR's? (mine is an 87).

It appears the 3/4" WAS available for the early Harley's. Is this the master cylinder to use when upgrading to dual Brembo's?

When using either of these, is DOT5 required in a NON ABS configuration?

Sorry if I repeat myself, just trying to get this clear. I have a set of dual Brembo's already but I currently have a single disk master cylinder on my FXR. I just want to make sure I get the correct parts to upgrade.

Greg
 

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I have to disagree with @kdkerr2 about piston size. A smaller MC piston will give you greater line pressure and more stopping power. Larger MC piston will give a wooden feel to the lever, with no modulation. Think of it this way, the force your hand puts on the lever is the same both ways, but the smaller piston will have less square inches, so there are more pounds per square inch than with a larger piston. I'd go with the stock Harley 11/16" setup.
 

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Sorry if I repeat myself, just trying to get this clear. I have a set of dual Brembo's already but I currently have a single disk master cylinder on my FXR. I just want to make sure I get the correct parts to upgrade.

Greg
If you are running stock switches/housing it's best to run the H-D 11/16" so it fits up and the brake switch works.
Check Ronnie's parts numbers for compatibility.
 

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Gregg55_99 asked: "
"Was the 11/16" master cylinder ever used with the dual disks for the early FXR's? (mine is an 87).
It appears the 3/4" WAS available for the early Harley's. Is this the master cylinder to use when upgrading to dual Brembo's?
When using either of these, is DOT5 required in a NON ABS configuration?"

I can't give a correct answer because I don't know. I assume that several of the FXR [like the Low Rider Sport] which ran dual disks used the 11/16" master cylinder. The 3/4" master cylinder posted in my Ebay add was an older model of master cylinder and there's some fitment problem with modern switch boxes [as Hillcat commented on]. Harley switched to DOT4 brake fluid a few years back when they realized that DOT5 was incompatible with antilock brake systems. On all your old brake systems [before the ABS] continue to use the DOT5. It's much easier on paint jobs! I'm running DOT5 in my upgraded custom brake setups and I'm aware of the compatibility problems of the DOT5 in my Tokico calipers. No problems with them so far but I can't recommend them to anyone else. If you are by chance upgrading to an antilock brake system make sure all the DOT5 fluid is thoroughly flushed out of your upper brake components. The two DOT fluids do not mix well!
 

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My red/white FXR has Brembo RCS master cylinders for the front brakes and clutch. They work very well with most four pot calipers found on sport bikes.

As for the remark about them not having a brake light switch, wrong. These MCs have a pressure switch that can easily be integrated into any oem or custom wiring harness.

This is my FXR with Brembo RCS master cylinders on the RB Racing website.

Scroll down.
RB Racing website with Turboprops FXR.

 

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Excellent! You are correct. I wasn't aware until recently that the Brembo RCS master cylinders came with an on board brake lite switch. So hooking up a hydraulic pressure switch for the front brake is unnecessary.
 

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Excellent! You are correct. I wasn't aware until recently that the Brembo RCS master cylinders came with an on board brake lite switch. So hooking up a hydraulic pressure switch for the front brake is unnecessary.
They also have the ability to mount the same switch on the clutch MC. Typically used for racing applications to trigger another event.
 
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