SMC slack adjustment

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I've tried searching and couldn't find the answer so here's a new thread. Over the last few days I've noticed a distinct "thunk" sound coming from the front end when I apply the rear brake. Obviously my first thought was that I had something loose. Second thought was that it was the SMC. Park the bike and jiggle the left caliper and sure enough, there is about 1/8 - 1/4 inch play from front to back. Seems like there should be an adjustment to take the slack out so it doesn't make this noise. Any ideas?

Thanks
 

ST Gui

240Robert
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There is no adjustment only service/replace.

I think John Heath measured the travel on a properly functioning SMC and the play was 1mm. Whatever the measurement is there's no adjust. (I hope that's right!)
 
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+1 on that,,, for there to be that much play,,, your smc must be jammed on. However,,, I am not sure that if it is jammed on,, it should be activating your rear brakes centre piston. Can you test its function by activating the smc by hand (push it towards the fork),, and feel the rear brake dragging when you spin the rear wheel with your foot (while laying beside the bike). Sounds weird,,, I know,, but I can lock my rear wheel this way. I also have only about 1mm travel on my recently cleaned smc. Let us know,, Cat'


There is no adjustment only service/replace.

I think John Heath measured the travel on a properly functioning SMC and the play was 1mm. Whatever the measurement is there's no adjust. (I hope that's right!)
 
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If you truly have 1/4 inch play in your SMC, the piston is stuck up inside the bore, and your on the verge of having your rear brake lock up on you without warning!
Search the threads for SMC replacement. Order the entire assembly instead of just the rebuild kit, as your bore is more than likely damaged and the rebuild kit would not fix it.
.02
 

dduelin

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I've tried searching and couldn't find the answer so here's a new thread. Over the last few days I've noticed a distinct "thunk" sound coming from the front end when I apply the rear brake. Obviously my first thought was that I had something loose. Second thought was that it was the SMC. Park the bike and jiggle the left caliper and sure enough, there is about 1/8 - 1/4 inch play from front to back. Seems like there should be an adjustment to take the slack out so it doesn't make this noise. Any ideas?

Thanks
Measure it. It's likely much less than this guess. These things make a little noise over bumps.
 
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The only ones that I've seen that make "noise" over bumps, is when there is air in the system, or the piston is not working as it should.
I would still strongly suggest correcting the situation and not risk an accident due to a part of the brake system not working correctly.
.02
 
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Question: what happens when you keep your foot off the brake pedal and apply the front brake only? Should be the same result I think, since activating either the front or rear brake lever/pedal applies braking to both wheels, even if the SMC is not functional. This is part of the LBS.
 

Kevcules

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Question: what happens when you keep your foot off the brake pedal and apply the front brake only? Should be the same result I think, since activating either the front or rear brake lever/pedal applies braking to both wheels, even if the SMC is not functional. This is part of the LBS.
I think you're right but the bike (front wheel) has to be turning so the braking action on the front rotor raises the SMC to activate the rear. Correct?
 
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That's correct, but if I am not mistaken applying the front brake applies one of the pistons in the rear caliper also, even if both wheels are stationary. I'm sure someone will correct me if I'm wrong on that point
 
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There must be 50 threads on this. Here's one from 12 years ago:

https://www.st-owners.com/forums/showthread.php?4968-Front-end-clunk&highlight=SMC+play

There is no air in my system and the system works just fine, thank you very much.
I'm thinking about flying into Florida in the next month or so to visit my grandson, want to bet a couple rib eye's and colds ones, that I won't find any air in your bike?
I will even bleed it for FREE, provide the DOT 4, and leave a MP tool for your toolbox as a door prize :rofl1:
Out in front of the school by the flag pole at three o-clock....I'm calling you out. :well1:
 

Kevin_56

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but if I am not mistaken applying the front brake applies one of the pistons in the rear caliper also
No brake fluid or lines flow from the front brake unit to the rear brake caliper. There are lines from the rear to the front calipers.

At no time do you ever have to bleed a rear caliper from the front brake unit.

Applying the front brake while the bike is stationary will not apply the rear brake caliper.
 
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Thanks but I'll pass.
What?...And pass up on a chance to meet the ST Whisperer in person, get your brakes bled for FREE, and get a motion pro brake bleeding tool....not to mention dinner and drinks if your right?
We could even set up a date, and invite other local st-owner members over for a mini brake bleeding demo etc....Ok, to witness the outcome...:rofl1:
 
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Dave, I really think you should take the opportunity to meet Larry. He really is a nice guy! Maybe you two could meet on neutral ground at my place....
 
OP
OP
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How easily should the piston compress? I pulled the caliper, removed the circle clip and removed the part that contacts the piston. The outer bore looks immaculate. Laying on the ground, I kept my finger on the piston and used my foot to push lightly on the rear brake. The piston pushed out a little and I was able to push it back in with some resistance. I figure that's probably normal since it's a hydraulic circuit. When I reassembled everything I stepped on the rear brake a few times and the slack was gone. I then tested the functionality by rotating the rear tire with my foot and pushing the caliper forward as it would if I were moving and used the front brakes. Stopped the rear as expected. Now the slack is back.

Might have to pull the piston and clean it up? Is it possible that there is just a little bit of air in the system causing it not to expand and retract as it should?
 
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You could have a few different issues going on here.
When testing the SMC with the spin the wheel and push on the caliper is a difficult thing to explain to folks. I've seen some folks take both hands and push hard enough to almost take the bike of the center stand and think because the wheel stopped that it passed the test! The SMC should only move 1-2mm at most, and you should be able to take just 1 finger or your thumb and GENTLY push forward to stop the wheel. You could push hard enough on a failing unit to still cause the rear wheel to stop.
The needle bearings at the lower pivot point, as well as the countersunk head of the bolt could be dry and binding also.
You say the bore looks good? Are you sure? Due to the angle of the piston, the bore normally wears on the inside top of the unit, and will appear to be a dark line, where the plate scores it.
Yes, air can and may cause some clanking or play in the SMC, the bleed procedure in the service manual should be followed correctly and in the proper sequence. You should also check out Jfheath's article on how to properly bleed the system and what to look for.
However, If you took the unit apart, this may all be a moot point, since Honda and the service manuel instructs you not to remove the assembly since the distance is set at the factory, and you will have no way of knowing if you reassembled it back to the factory position. Did you replace the C-clip correctly with the sharp edge pointing up? I ask because another member had his grenade on him and come apart after he fiddled with it.
My guess is this: Your SMC is/was on its way out, and that is why the piston is/was sticking. However to error on the side of safety (since it is a brake system) I would replace the entire unit due to the disassembly, and then lubricate the needle bearings, replace all the crush washers, and have the entire system bled correctly just to be safe.
YMMV
 

Kevcules

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How easily should the piston compress? I pulled the caliper, removed the circle clip and removed the part that contacts the piston. The outer bore looks immaculate. Laying on the ground, I kept my finger on the piston and used my foot to push lightly on the rear brake. The piston pushed out a little and I was able to push it back in with some resistance. I figure that's probably normal since it's a hydraulic circuit. When I reassembled everything I stepped on the rear brake a few times and the slack was gone. I then tested the functionality by rotating the rear tire with my foot and pushing the caliper forward as it would if I were moving and used the front brakes. Stopped the rear as expected. Now the slack is back.

Might have to pull the piston and clean it up? Is it possible that there is just a little bit of air in the system causing it not to expand and retract as it should?
Do you mean that there is a lot of pedal travel before you feel resistance and brakes? I've got that with my set up and I haven't touched the brakes yet. I've got a more than normal amount of travel before the rear brakes work, but if I hit the pedal a few times, the slack is gone for that stop. Is that normal? Does that mean the brakes are worn or the that the caliper is retracting normally?
 
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