How to fix those Jumpy, Jerky front brakes - Road Star Warrior Forum : Yamaha Star Warrior Forums

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Old 11-26-2019, 07:13 AM   #1 (permalink)
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Default How to fix those Jumpy, Jerky front brakes

Fix the Jerky, jumpy, Smooth then suddenly abrupt braking when applying the Front brake lever lightly to gradually harder.
A few issues causing this is between the Front brake lever and master cylinder. There's temporary fixes and more or less permanent.

Reasons for the issue-
1. The master cylinder plunger face is worn and has a divit from the lever adjuster bolt.
2. The front brake lever pivot bolt has worn the lever pivot bolt hole to an ovaled shape.
3. The master cylinder piston is catching inside.


The 1st possible reason for the braking issue is the Plunger has a divit worn in from use of the brakes or vibration(Divit can be shallow or deep). In return causes the lever adjuster bolt tip to bind in the divit on the plunger face then release. Depending how worn or how much pressure is applied to the brake lever you'll feel different results of intensity of the jerky jumpy brakes.



These 2 pics show a deep divit from the adjuster bolt tip.
Other divits can be more shallow and cause the same affects.



Heres a temporary and more or less permanent fix to the master cylinder plunger worn face.
Of coarse the best way to fix it is rebuild the master cylinder with new parts in the pic below but chances are with the temporary fix it's going to happen again soon.



Temporarily fix-
To temporarily fix this jumpy issue from the warn plunger face is to spin the plunger 180.
Doing this will get your brake feel back to normal for a while. This is because the plunger face has a worn divit from the lever adjuster bolt tip. The wear of this divit is directional from the center of the plunger face towards the from of the bike. When spinning the plunger 180 it kind of resets a new area for the adjuster bolt tip to slid while pulling the brake lever.
The worn plunger face causes the lever to Jump due to the divit made from the adjuster bolt. The adjuster bolt tip binds in the divit with the more force applied to the lever.
Turning the plunger 180 may not help if the divit is too deep but the next step will fix that.




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Last edited by rij1; 11-26-2019 at 08:59 AM. Reason: Adding info
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Old 11-26-2019, 07:14 AM   #2 (permalink)
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Permanent/long term fix-

This is the more permanent fix in a 2 part mod.
I have done this on multiple motorcycles.

Step 1-
This part is to resurface the plunder face as the plunger is solid aluminum.
You can do this 2 ways on or off the bike. The first is to get a small flat medium coarse file and fine coarse flat file.
It has to be narrow enough to fit between the mounting area where the brake lever mounts. Basically narrower then the thickness of the lever pivot area. A 3/8" wide file fits loosely enough to work.



The first thing you wanna do is push the dust seal back exposing more of the plunger face otherwise you'll ruin the dust cover seal.



Depending on how warn the face is or how deep the divit is you'll wanna pick the correct coarse of file to use as if the divit is deep you'll want a medium file then finish with a fine file.
When filing you will want to try and keep the file flat to the face as possible and keep checking that the whole surface is having material removed in the direction of your strokes. Don't apply to much pressure.
Once you see the whole surface has file marks grab a pair of needle nose pliers and turn the plunger 1/4"- 1/2" a turn.

Then continue with the filing and repeat the steps of turning and filing. If you start off turning 1/4" them repeat and the same if you start with 1/2" turn.
Keep an eye on the face as you'll wanna stop when the divit is completely gone. Once you get to the point where the divit is almost gone you'll want to use the fine file and finish with light pressure.



Once the divit is gone you will want to carefully remove the dust cover seal to get all the aluminum filings out and cleaned away. Once it's all cleaned off install the dust seal cover and move on to the next step. This step is Not needed but will prolong the life of your plunger face, lever application and brake feel.








The other way to resurface the plunger face is to remove the plunger. You'll want to remove the brake fluid from the resovoir. After you have the resovoir empty you'll want to remove the dust cover seal from around the plunger. After it's removed you'll see an E-clip inside holding the plunger from coming out.



Remove the E-clip with snap ring pliers or very small tip needle nose pliers. Have a rag handy as some brake fluid is in there and you do not want to get it on your paint. Once the E-clip is removed carefully pull the plunger out. Careful with the rubber seal on the plunger.
Clean the fluid off the plunger and take the steps above to file the plunger face down.



Once the plunger is done remove all the filings. When installing the plunger apply some brake fluid to it and the seals then slowly work it in.

This pic shows a roughly filled down plunger face i did a year ago but did not a file smoothened adjusting bolt tip hence the reason I figured out the next step.



Step 2-
Smoothing out the brake lever adjusting bolt tip.
The bolts machining job is not so great and has flaws but there easy to fix. There's 3 small issues with the bolt tip that contacts the plunger face.
1, there's a small pit right at the tip of the bolt.
2, the domed area on the tip has swirls.
3, the domed tip is to pointy.

The pit in the tip of the bolt is not smooth and lightly cuts,scraped, digs into the plunger face.
The swirls scrape and catch the plunger face depending how deep the tip eats into the face.
The pointy dome of the bolt has a smaller then wanted contact foot print when contacting the plunger face. The smaller the foot print the more pressure in that tiny area and with steel to aluminum it's not good as it'll create divit easier.

When fixing this issue you'll want to smooth down the pit on the tip of the bolt and smooth down the swirls on the rest of the domed tip area. Doing this will reduce the pitch of the dome creating a flatter and bigger contact foot print to the plunger face thus spreading load pressure to reduce divit in the plunger face.

Use a fine file and just file the very tip of the bolt where the pit is. Completely remove the pit but stop there. Next file the swirls on the rest of the domed tip. File in a rocking and spinning way so you don't create flat spots. Don't file the threads.
Once the bolt tip is nice and smooth all around you can install it back on the lever.











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Old 11-26-2019, 07:15 AM   #3 (permalink)
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Side note, your levers have/had a spring to push the lever back keeping the lever adjusting bolt off the plunger face when not applying the brake.
If you don't have the spring anymore you'll want to adjust the lever bolt tip as close as possible to the plunger face as if you have to much of a gap you'll create a hammering affect between the bolt tip and plunger face from the vibration of the Road and engine which will cause premature wear creating a deep divit.
If you still have the brake lever spring use the manual stated lever end play of 2-5mm(0.08-0.20in)








The 2nd possible reason for the jumpy braking is the front brake lever pivot bolt has worn the lever pivot bolt hole to an oval like shape.
This causes the steel bolt to bind inside the worn aluminum oval section in the lever. Or causes the lever to pivot in the normal pivot hole with light lever pull then jumps/slides from the normal hole position to the worn oval area when more force is applied to the lever. The fix to this is to modify the lever pivot bolt hole by boring the hole with a drill bit then pressing a sleeve bearing. This provides smooth use and will last.



The 3rd possible reason is if the plunger/piston is binding due to damage and the only fix is to rebuild the master cylinder.





If there's anything I should add let me know. I'm sure I'm missing something as its very late and I'm tire haha.
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Old 11-26-2019, 10:52 AM   #4 (permalink)
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This is great information, assembled all in one place I think for the first time, well organized and looks complete enough to help members identify any additional search criteria for identifying and fixing this range of troubles well into the future. You do great work, this is awesome.
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Old 11-26-2019, 06:24 PM   #5 (permalink)
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Quote:
Originally Posted by arizonawarrior View Post
This is great information, assembled all in one place I think for the first time, well organized and looks complete enough to help members identify any additional search criteria for identifying and fixing this range of troubles well into the future. You do great work, this is awesome.
I figured it should help out many as I see posts here and Facebook regarding this braking issue. I kept it on the brake lever area to keep it a bit simple even though the actual brake calipers could cause issues but atleast this will get this area situated.

Btw, I tried to change the title name but it didn't seem to work. When looking at the thread list it still shows 'Jumpy brake fix' while in side the thread it says ' How to fix those Jumpy, Jerky front brakes' I would like it to say 'How to fix those Jumpy, Jerky front brakes' in both the thread listing and thread if you can help with that.
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