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Secondary Master Cylinder Query
- Thread starter jfheath
- Start date
I believe its normal. I know if I am just moving the bike forward a little bit and gently tap the brake handle repeatedily the clunk will be heard everytime but only when going forward. It will not "clunk" going backwards.
A couple things to check, first remove the left caliper by removing both the top and bottom caliper mounting bolts, gently move it out of the way. You'll notice the top bolt rides inside a brown metal/wire mesh type collar, feel with your finger, and look at the backside of this collar, sometimes they get pinched or folded when folks put the caliper yoke back on. This may be a cause. Also pull the bushing out of the lower mounting point and take some scotchbright (dishpad) and clean and polish it. Your looking to see if you can see any scorring or gaulding from wear on the bearings, the poke your pinkie finger inside the hole and see if your bearings are dry or damaged. I've found them in both conditions, and even had loose needle bearings inside causing this type of noise.
Lastly, the "lay on the ground like your drunk and spin the wheel with your foot test" will show that the unit will stop the wheel, and then release it, but it does not always show the condition of the bore (minor pitting in the area of the flat disk that the c-clip snaps down onto), and like you said there may be some damage there, and what you may be hearing are the parts settling back down into position again. There is only a couple milimeters of travel there to begin with. As you push the unit up and forward, it should feel smooth and under pressure, not "click, forward and click back" if that makes sense.
If all these areas prove to be in good serviceable condition, I'd say you may just be hearing the spacer against the springs in your front shock.
Hope this helps.
Good luck, and let us know what you find.
Igofar
Lastly, the "lay on the ground like your drunk and spin the wheel with your foot test" will show that the unit will stop the wheel, and then release it, but it does not always show the condition of the bore (minor pitting in the area of the flat disk that the c-clip snaps down onto), and like you said there may be some damage there, and what you may be hearing are the parts settling back down into position again. There is only a couple milimeters of travel there to begin with. As you push the unit up and forward, it should feel smooth and under pressure, not "click, forward and click back" if that makes sense.
If all these areas prove to be in good serviceable condition, I'd say you may just be hearing the spacer against the springs in your front shock.
Hope this helps.
Good luck, and let us know what you find.
Igofar
- Joined
- Feb 11, 2006
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- GL1800 R1200RT NC700
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I was going to say check the clevis pushrod that connects the SMC to the fork leg. IIRC the pushrod passes through a metal washer that has a rubber seal bonded to it. I think miles and age get to the rubber and let the pushrod tap against the washer. It doesn't affect braking or leak fluid because it is not a fluid seal but it does cause a clunking.
After watching the video that is exactly the clunk I hear. I also spray some anti corrosion stuff down into the rubber boot on top of the SMC...is that a good idea? I use BOESHIELD made by boeing. They use it for anti corrosion on aircraft.
You are better off just replacing the SMC. There is potentially a lot of damage in the SMC that seals and rubber won't fix. Plus it's a bear to take apart. 2 hours to put a kit in. 10 minutes to replace.
Throttlejockey
Padden is my hero
Yes, that circlip holds the assy together. That looks like quite a bit of movement to me.
BTW- I would not recommend spraying anything into the boot area. There is an o-ring seal on the cover that the circlip holds in. If lubricant got in there I don't think it would be good for the seals. Silicone grease only should be used.
Hate to say it, but yours does have quite a bit of moment. The only two things in my mind that would cause that are corrosion/wear causing the spring to not return the piston smoothly, or air in the system 
but knowing that you probably did a really good job bleeding everything, I'm guessing its the weak spring and corrosion inside the SMC. I'd replace the entire unit as a whole, as it is much easier to simply bolt on a new complete unit, and does not cost that much more....or....you can purchase the rebuild kit, open the old one up and find the bore damaged where the plate sits (common) and find out your shiny new rebuild part will not work in the old bore and that you've wasted your money. .02
but knowing that you probably did a really good job bleeding everything, I'm guessing its the weak spring and corrosion inside the SMC. I'd replace the entire unit as a whole, as it is much easier to simply bolt on a new complete unit, and does not cost that much more....or....you can purchase the rebuild kit, open the old one up and find the bore damaged where the plate sits (common) and find out your shiny new rebuild part will not work in the old bore and that you've wasted your money. .02
Throttlejockey
Padden is my hero
You'll know once you get the circlip off and the piston out. It will be obvious if it's bad.
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Every so often I peel back the boot and check mine for corrosion where you can see. If there is none present and the SMC compresses and extends freely and the action of the SMC imparts braking action to the rear cailper there should be no damage inside the bore. How would you know? Well, under braking the pressure in the bore is several hundred PSI. That is going to force brake fluid past a damaged or worn seal piston seal. If there is no such leakage and the SMC piston moves in and out and is not seized and the brakes work normally you can figure out the rest.
When I had up to maybe 70,000 miles the regular brake fluid flush and bleed would remove the clunking, at least for a few thousand miles but it would return. At some point regular maintenance would not remove the clunk it was there all the time. I bought the replacement parts for the system but it continued to work normally and still does.
When I had up to maybe 70,000 miles the regular brake fluid flush and bleed would remove the clunking, at least for a few thousand miles but it would return. At some point regular maintenance would not remove the clunk it was there all the time. I bought the replacement parts for the system but it continued to work normally and still does.
Thanks for that John....it's nice to know that I'm not the only one who has suffered from bubbles.
Scooter
This space for rent...
John, some questions just to clarify for me:
1) What type of vacuum pump do you use? A hand operated one or is it motor or pneumatic driven?
2) In step 2 you state "the double action of the SMC being activated and the vaccuum tube ensuring that the fluid is sucked through as well as being pushed". Now normally I would have been pumping the rear brake pedal to help push the fluid through at this point but it sounds like you are pushing the SMC piston in and out, correct? And if so, how many times did you have to cycle the SMC piston to move the amount of fluid out that you did? Is that hard on your hand to do?
3) In step 3 you clear our both of the rear bleeder valves. Normally the manual wants you to bleed the rear center bleed valve then the center bleed valve on both front calipers before you finally connect up to the rear outer bleed valve. Did you simply skip doing the front valves again at this point or just didn't mention it?
Again in step 3, did you push to fluid through the system using only the SMC piston instead of the rear brake pedal?
Roughly how much travel is there in the SMC piston when you are pumping it?
Sorry if it sounds like I'm being an Inquisitor here, just want to know how you did this for future reference. Kind of wish you'd done this last week since I could have tried this myself this past weekend.
:yes:Thanks...
I've found and removed air bubbles at the smc banjo fitting on a couple difficult to bleed bikes as well. I didn't have to use a vaccum bleeder however, or push the plunger as you did, I simply used my MP bleeder and cracked the banjo fittings open and burped the air out that way instead. Both ways will work. On some bikes (HD) the master cylinder housing up on the bars, has an investment casting that can harbor a very large air bubble, despite any efforts to bleed it, including using a vaccum pump, the only way to get at it is to loosen the bolts and tip the entire casting while tapping on the housing. After a while, you just figure out where all the problem areas are, and Lord knows our ST's have alot of places for air and dirty fluid to hid it.
Glad you were able to finally correct your system. Good job, worthy of the MOTM for sure
Glad you were able to finally correct your system. Good job, worthy of the MOTM for sure
pdfruth
P.D.Fruth
Just to be clear :BDH:, after all this, can you confirm you NO LONGER have any of that "clunking" you've shown in the video in post #7... correct?
I've got the exact same thing going on, and running out of hair (nearly pulled it all out at this point)
pdfruth
P.D.Fruth
Finally... success !
There was a good sized pocket of air, in the rear-outer caliper piston bores. I pushed the pistons all the way back into the caliper block, while watching fluid in the bleeder tube. Shazam, a big air bubble. Have no idea how it got in there. I've never opened the lines in that part of the hydraulic system.
So, I'm good. I too can make the "clunck" happen if I grab hold of the SMC and push it back and forth. But there is only a 2mm - 3mm movement now. Better than before.
Holy cow, the rear brakes on the ST have incredible stopping power. I've gotten so used to the [barely useful] rear brake on the track bike. You can stand on the track bike's rear brake pedal and barely notice any decel. The ST's rear, on the other-hand, is another story. Barely touch that brake pedal, and your stopped !
Thanks for your write-up J.
There was a good sized pocket of air, in the rear-outer caliper piston bores. I pushed the pistons all the way back into the caliper block, while watching fluid in the bleeder tube. Shazam, a big air bubble. Have no idea how it got in there. I've never opened the lines in that part of the hydraulic system.
So, I'm good. I too can make the "clunck" happen if I grab hold of the SMC and push it back and forth. But there is only a 2mm - 3mm movement now. Better than before.
Holy cow, the rear brakes on the ST have incredible stopping power. I've gotten so used to the [barely useful] rear brake on the track bike. You can stand on the track bike's rear brake pedal and barely notice any decel. The ST's rear, on the other-hand, is another story. Barely touch that brake pedal, and your stopped !
Thanks for your write-up J.
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