rear brake master swap. - Wrist Twisters
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post #1 of 20 Old 01-20-2020, 11:54 AM Thread Starter
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rear brake master swap.

Has anyone ever swapped out the 919 rear brake master for one of a different model Honda.
I've never liked the feel of the rear brake on my bike. Too long is the stroke for my liking. Too much free play.
I've noticed the master cylinder is small when compared to other similar sized Hondas.
The 919 has a 1/2 inch cylinder. The cbr600 is 14mm and 5/8. The cbr900 has a 5/8 and the blackbird is 11/16.
I'm thinking of going for a 5/8 cylinder swap. At approx 15mm it's about a 50% increase in area size. I'll lose feel, but will have to move the brake petal less to get brake to slow down/lock up.
Any thoughts?

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post #2 of 20 Old 01-20-2020, 01:36 PM
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Quote:
Originally Posted by Islandboy View Post
Has anyone ever swapped out the 919 rear brake master for one of a different model Honda.
I've never liked the feel of the rear brake on my bike. Too long is the stroke for my liking. Too much free play.
I've noticed the master cylinder is small when compared to other similar sized Hondas.
The 919 has a 1/2 inch cylinder. The cbr600 is 14mm and 5/8. The cbr900 has a 5/8 and the blackbird is 11/16.
I'm thinking of going for a 5/8 cylinder swap. At approx 15mm it's about a 50% increase in area size. I'll lose feel, but will have to move the brake petal less to get brake to slow down/lock up.
Any thoughts?
There is no question that there is cumulative slop along the entire linkage chain.
Having said that though, I find with mine that I get some initial lost motion that doesn't take very much pedal travel, and once the pads load up, not much lever travel is needed for the actual braking.
Mind you I am a light rear braker.
It sounds as though you are using more travel in the realized braking effect zone, or use yours harder.
Personally, I'd be leery of going with less hydraulic ratio in an effort to reduce travel, but your situation may be entirely different.
By the way, I have pondered stiffening the lever some day, as if one is hard on it, some flex can be felt.
Anyway, it would be an interesting experiment to see how a diameter master would do.
A 50 % change is mega.
The 14 mm would net a 21% change, which in itself is major.
Plus look for any lost motion factors.
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post #3 of 20 Old 01-20-2020, 01:53 PM
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post #4 of 20 Old 01-20-2020, 02:09 PM Thread Starter
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Thanks mcromo. I'm pretty much slop free. Using Sato rearsets.
I use my rear brake a lot and it shits me having to take up the freeplay, about 15 to 20mm.
I'll report what I find when I install a 5/8. See if it shortens my stroke.
I agree 50% is massive.
Why was such a small diameter, 1/2, rear master used when similar machines at the time are using larger diameter cylinders?
Was it too give more feel to the rear brake on a street machine? Perhaps to lessen the risk of rear lockup on the street?

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post #5 of 20 Old 01-21-2020, 12:07 AM
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Seriously, how much braking power can one get with that setup, as compared to foot actuated?
Do they use more ratio (smaller master piston) and the greater lever stroke is more easily coped with by the thumb?

Is your setup on the R bike a dual?
As in thumb or foot actuation?

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post #6 of 20 Old 01-21-2020, 12:18 AM
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Quote:
Originally Posted by Islandboy View Post
Thanks mcromo. I'm pretty much slop free. Using Sato rearsets.
I use my rear brake a lot and it shits me having to take up the freeplay, about 15 to 20mm.
I'll report what I find when I install a 5/8. See if it shortens my stroke.
I agree 50% is massive.
Why was such a small diameter, 1/2, rear master used when similar machines at the time are using larger diameter cylinders?
Was it too give more feel to the rear brake on a street machine? Perhaps to lessen the risk of rear lockup on the street?
Hmmm, mine has much less free play than yours.
Maybe 1/2 of what you are describing.

As for the OEM spec, my guess is that most street riders of 919 like bikes, prefer a softer pedal and are oblivious to the travel.
The smaller piston reduces lever effort for any given braking power, but requires more travel.
Lockup is actually easier, as less pedal force is needed.
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post #7 of 20 Old 01-21-2020, 08:40 AM
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Quote:
Originally Posted by mcromo44 View Post
Seriously, how much braking power can one get with that setup, as compared to foot actuated?
Do they use more ratio (smaller master piston) and the greater lever stroke is more easily coped with by the thumb?

Is your setup on the R bike a dual?
As in thumb or foot actuation?



Not about power, it's all about modulation and precision. More times than not the rear wheel is not even touching the ground and locks up quite easily with a 13mm piston. You simply get more control over what the rear brake is doing with the thumb.


I skipped the rear foot pedal altogether, but I can piggyback the lines into the foot master if someone does want to be retain both thumb and foot operation.
http://www.rogueracing.org/zx10r/thumbbrake.htm
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post #8 of 20 Old 01-21-2020, 09:28 AM
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Quote:
Originally Posted by LDH View Post
Not about power, it's all about modulation and precision. More times than not the rear wheel is not even touching the ground and locks up quite easily with a 13mm piston. You simply get more control over what the rear brake is doing with the thumb.


I skipped the rear foot pedal altogether, but I can piggyback the lines into the foot master if someone does want to be retain both thumb and foot operation.
ROGUE RACING Thumb Brake install
Eyeballing things, I can see how the lever length of the thumb kit is way less than the foot kit.
I suppose it translates to the ergonomic range of thumb being a good match for the combination of overall ratio from piston diameter and lever length and resultant stroke.
The bonus is the precision, plus the ability to actually get at the thumb lever under all positions of lean, plus any right foot dangling!

What piston diameter is in the kit? 14 mm?

I also like the mega weight saving by the back hogout of the back of the lever operating face.

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post #9 of 20 Old 01-21-2020, 09:31 AM
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13mm

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post #10 of 20 Old 01-21-2020, 12:51 PM
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One thing you may want to take a look at is the significant distance from the brake pedal actuation arm to the actual heim joint for the master. A distance of ~25mm is certain to cause at least some flex of the pedal actuator arm. Unfortunately there is little you can do about that.

An increase of master cylinder piston diameter from 12.7mm (1/2 inch) to 15.875mm (5/8 inch) would decrease the overall ratio by 56%, meaning you would have to literally put all your weight on the pedal to get any significant braking. Not good. At least the pedal would be nice and firm.

A 14mm master would decrease the ratio by 21.5%, better, but effort would be that percentage greater. That is if it will bolt up without major modification to the rearset.

For reference I checked my rear brake, which I rarely use, and found that the movement of the pedal before any discernible braking occurs is around 15mm, and lockup maybe another 10mm. If yours is significantly greater I'd suspect either the Sato rearset brake pedal is dimensionally different that the stocker. Unlikely, but possible. Or there is an air bubble at the high point of the system, which is the banjo fitting on the master cylinder that no amount of "pump up, hold, crack the bleed screw, close, repeat" will address due to the small amount of fluid moving. Perhaps a banjo bolt with an integrated bleed fitting would be beneficial, but there is an alternative: remove the clevis pin, unbolt the master cylinder and the reservoir, pivot the whole assembly up until the reservoir hose is the high point, then retract the caliper piston. This will force any air out of the banjo fitting and into the reservoir. That is one of the reasons why I always use AN fittings instead of banjos: bleeding is much easier due to no convolutions in the fluid path.

One more thing you can do is fettling the master cylinder. Stock, the rear primary piston seal at rest position is at least a millimeter or more from the compensating port, meaning the first 5 to 8mm of pedal travel is used to simply cover the port to begin pressurizing the system. Normally when I am building a master I position the primary seal to occlude about 10% of the port using thin shim washers. Pressurization starts almost immediately. It's not easy, but worth it when on the track, and in my case on the street.

And of course there is the Brembo thumb brake setup mentioned by LDH. Undoubtedly a perfectly functional solution, but at a retail price of $2000.00 it's also a perfectly unreachable solution for the average street rider. Now if you got one free for "testing purposes" it'd be a good way to go. That is if it can be mounted to the bar instead of the fork.

My mistake: the Sato thumb brake is listed at $650. At one third the price of the Brembo it's still ... pretty expensive.

Rob
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post #11 of 20 Old 01-21-2020, 01:00 PM
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Meh... Thumb Brake kit is about $750 all in with the custom line and Reservoir etc using the existing rear caliper. If you want to piggyback the original rear master instead of deleting it budget another $120

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post #12 of 20 Old 01-21-2020, 01:35 PM
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Quote:
Originally Posted by robtharalson View Post
One thing you may want to take a look at is the significant distance from the brake pedal actuation arm to the actual heim joint for the master. A distance of ~25mm is certain to cause at least some flex of the pedal actuator arm. Unfortunately there is little you can do about that.

An increase of master cylinder piston diameter from 12.7mm (1/2 inch) to 15.875mm (5/8 inch) would decrease the overall ratio by 56%, meaning you would have to literally put all your weight on the pedal to get any significant braking. Not good. At least the pedal would be nice and firm.

A 14mm master would decrease the ratio by 21.5%, better, but effort would be that percentage greater. That is if it will bolt up without major modification to the rearset.

For reference I checked my rear brake, which I rarely use, and found that the movement of the pedal before any discernible braking occurs is around 15mm, and lockup maybe another 10mm. If yours is significantly greater I'd suspect either the Sato rearset brake pedal is dimensionally different that the stocker. Unlikely, but possible. Or there is an air bubble at the high point of the system, which is the banjo fitting on the master cylinder that no amount of "pump up, hold, crack the bleed screw, close, repeat" will address due to the small amount of fluid moving. Perhaps a banjo bolt with an integrated bleed fitting would be beneficial, but there is an alternative: remove the clevis pin, unbolt the master cylinder and the reservoir, pivot the whole assembly up until the reservoir hose is the high point, then retract the caliper piston. This will force any air out of the banjo fitting and into the reservoir. That is one of the reasons why I always use AN fittings instead of banjos: bleeding is much easier due to no convolutions in the fluid path.

One more thing you can do is fettling the master cylinder. Stock, the rear primary piston seal at rest position is at least a millimeter or more from the compensating port, meaning the first 5 to 8mm of pedal travel is used to simply cover the port to begin pressurizing the system. Normally when I am building a master I position the primary seal to occlude about 10% of the port using thin shim washers. Pressurization starts almost immediately. It's not easy, but worth it when on the track, and in my case on the street.

And of course there is the Brembo thumb brake setup mentioned by LDH. Undoubtedly a perfectly functional solution, but at a retail price of $2000.00 it's also a perfectly unreachable solution for the average street rider. Now if you got one free for "testing purposes" it'd be a good way to go. That is if it can be mounted to the bar instead of the fork.

My mistake: the Sato thumb brake is listed at $650. At one third the price of the Brembo it's still ... pretty expensive.

Rob
Thanks for repeating about your shimming of the seals to take up wasted travel.
I had forgotten about that gem of a tidbit.

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post #13 of 20 Old 01-21-2020, 01:37 PM
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Quote:
Originally Posted by robtharalson View Post
My mistake: the Sato thumb brake is listed at $650. At one third the price of the Brembo it's still ... pretty expensive.

Rob

Yea, but it is only money and you can always get more of that...




I wasn't originally going to put a thumb brake on the R6, but I'm thinking it is just begging for one
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post #14 of 20 Old 01-21-2020, 02:04 PM
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Quote:
Originally Posted by Islandboy View Post
Has anyone ever swapped out the 919 rear brake master for one of a different model Honda.
I've never liked the feel of the rear brake on my bike. Too long is the stroke for my liking. Too much free play.
I've noticed the master cylinder is small when compared to other similar sized Hondas.
The 919 has a 1/2 inch cylinder. The cbr600 is 14mm and 5/8. The cbr900 has a 5/8 and the blackbird is 11/16.
I'm thinking of going for a 5/8 cylinder swap. At approx 15mm it's about a 50% increase in area size. I'll lose feel, but will have to move the brake petal less to get brake to slow down/lock up.
Any thoughts?
See Rob's comment about master cylinder fettling.
You might want to consider checking your primary seal : port position at rest, and do some, aye?

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post #15 of 20 Old 01-21-2020, 02:15 PM Thread Starter
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Thanks Rob. Before buying a larger size master cylinder I will try all you have suggested. The bleeding and the shims.
Would reverse bleeding using a syringe help?
The shim idea sounds like the ticket. Would like to reduce the amount of lever travel before the pads load up.

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post #16 of 20 Old 01-21-2020, 02:44 PM Thread Starter
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OK...dumb question time. I'm staring at a stripped down master cylinder.
Where do the shims go? M8, under the mushroom head of the push rod?
I need to get washers less than 1mm thick?
Also Rob I got about the same travel you did.
Ta.

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post #17 of 20 Old 01-21-2020, 05:06 PM
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Quote:
Originally Posted by Islandboy View Post
Thanks Rob. Before buying a larger size master cylinder I will try all you have suggested. The bleeding and the shims.
Would reverse bleeding using a syringe help?
The shim idea sounds like the ticket. Would like to reduce the amount of lever travel before the pads load up.
A syringe will be ineffective at eliminating an air bubble due to the fact that the air may be moved into the master cylinder bore temporarily, but regardless of the amount of fluid pushed through gravity and the propensity of air to rise to the top of any liquid will bring your efforts to naught. The bottom line is you must take advantage of basic fluid physics and move the air to the port. Anything else is a waste of time and brake fluid.

I default to brass shim stock 0.005 inch (0.13mm) thick until I have the information necessary for adjustment.
The piston shimming procedure is relatively simple: with the master mounting ears gently fixed in a vice, the clevis removed but the fixing nut run all the way in, and the rod fed through a piece of metal or wood with an 8.2mm hole drilled clamped to the vice as a guide to enable accurate measurement of its height. While gently pushing against the rod to seat it measure the height four times, making a note of each number. Then feed low pressure air, below 10 PSI (0.7 Bar) into the outlet and listen to the sound of air coming out of the port, then run the nut out until it contacts the other side of the guide and continue to turn it slowly until the air sound stops completely and measure the height of the rod. Do that at least four times and subtract the smallest number from the at rest measurement. Add the size of the port, usually 0.2mm. That is roughly the thickness of the shim(s) needed. Install the shims and repeat the measurements until the port is closed at slightly more than 0.20mm movement. The shims are placed between the circlip and the washer on the pushrod.

It may take a few tries at first, but with experience things go considerably faster.

Bahati njema!

Rob
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post #18 of 20 Old 01-21-2020, 08:57 PM Thread Starter
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Thanks again Rob. Clear as mud!
I'll have a crack at this. Finding the correct shims will be a joy.

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post #19 of 20 Old 01-22-2020, 08:56 AM
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Quote:
Do that at least four times and subtract the smallest number from the at rest measurement. Subtract the size of the port, usually 0.2mm. That is roughly the thickness of the shim(s) needed. Install the shims and repeat the measurements until the port is closed at slightly more than 0.20mm
Correction to my previous post in red.
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Originally Posted by Islandboy View Post
Thanks again Rob. Clear as mud! But it does cover the ground!
I'll have a crack at this. Finding the correct shims will be a joy.
Go here:https://www.mcmaster.com/shims.
Invariably they will need trimming for the proper O.D., but with creativity it's easy to do. Remember that the only critical dimension is the thickness. Everything else just has to be good enough to be retained by the circlip and clear the pushrod (plus some). Shims can be stacked for the proper thickness, but the fewer shims in the stack the better. If you find the clearance is a little too small simply sanding the thickest shim in the stack will get you right where you need to be.

Good luck!

Rob
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post #20 of 20 Old 01-22-2020, 09:40 AM
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pretty bad ass mod imo.
Does the it go on the left or on the right bar? seems like on the right you'd have sooo much stuff to worry about; or i'm just a noob and can't handle another thing :P

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