SMC Question

Joined
Jul 8, 2007
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40
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Vancouver BC
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'06 non-ABS
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3657
After going through the threads here about SMC problems and rear brake lockup, one thing I notice is the lack of movement of the SMC when someone has had a problem related to this. Yesterday, my rear brke locked solid as I entered a parking lot so my assumption was the SMC had failed (again) I replaced the SMC assembly in 2011 with a brand new complete unit and have had no problems until now. My question is this: If the brake system and SMC are working properly, should I be able to move the SMC fore and aft on it's pivots easily? My feeling is there should be hydraulic pressure on the SMC so I should not be able to move it back and forth easily. One thing I noticed was when I started looking at the bike the SMC rocked back and forth quite easily with no resistance. By the time the tow truck got there my bike would roll quite easily, almost like the pressure had bled off and the brake released itself. I rode down the parking lot and applied only the front brake and I had exactly the same problem, rear brake locked solid. I'm hoping someone here with a bike that is working properly can tell me how much resistance the SMC has when it is pushed forward. I have not taken anything apart as yet, one thing I did read here was a possible brake pad popping out of the caliper on the end opposite the pin. Thanks in advance for your help and advice.

Mike
 

SteveST1300

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When mine failed it had a lot of movement but my back wheel would only lock when I used the brake pedal, if I just used froont brake lever I had no problem. I once had to open the rear bleed valve to release it. Sounds like your SMC is shot.
 

dduelin

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A functioning SMC should have very little free movement - perhaps 2 to 4 MM. With the bike at rest it takes a bit of force to move the entire caliper/SMC assembly forward and apply the rear brake. With the force released the assembly moves almost imperceptibly back into the relaxed 'brakes off' position.

If the assembly just moves back and forth with no spring back the SMC piston may be frozen down in the bore of the cylinder.

Was the new SMC installed and bled properly to begin with?
 

Reginald

cyclepoke
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I just fixed multiple problems with my breaking system, mostly SMC. Some of which the local shop caused and some caused by a bad pivot bearing.
1) The pivot bearing which allows the SMC to move fore and aft failed. The bearing delaminated the chrome and caused burrs in the inner race causing the SMC to lock up. I took the bearing race out, sanded it removing the burrs, cleaned the bearings with brake cleaner, repacked the bearing with EP lithium grease, and reassembled. It rotates smoothly now. I will see how this works over time; may have to replace it.
2) The SMC plunger under the boot had dried up leaving a concrete looking substance under the boot. I cleaned this up with brake fluid and packed the boot with brake caliper grease to prevent water and air from getting under the plunger. The SMC now moves fore and aft about 3mm and stops the rear wheel from spinning when compressed.
3) The brake system was improperly flushed and bled. I followed the instructions on this site, IGOFAR, and SM for bleeding. The proportional control valve bled black fluid because the shop did not flush it as they should have. This is the line leading to the SMC.
4) The shop used EBC brake pads which are thicker than the OEM brakes and pushed the caliper pistons almost flush to the caliper. I haven't replaced them yet due to budget; spent it on poor work from shop. They rub more than they should. The rotors are not hot while riding, but I like my front wheel to spin more freely when off the ground.
5) I cleaned the brakes according to Joe's article on this site. The pistons weren't bad but the pins had some build up, possibly not releasing the pads properly. I also put a dab of brake grease on the retaining pads of all calipers.

Hope some of this helps.
 

T_C

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I rode down the parking lot and applied only the front brake and I had exactly the same problem, rear brake locked solid.
Umm.. yeah.. that is when the SMC activates, when you apply front brake. The front rotor will drag the pads and rotate the left front caliper forward, depressing the SMC.

Now you may have been able to apply rear brakes only, but even then with linked brakes (rear pedal activates front low piston) it could be enough to grab and depress the SMC a bit.
 

dduelin

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The SMC is always immediately suspected when the rear brake drags but the rear caliper may not be sliding freely on the pins due to corrosion or lack of grease on the pins or the pads could be improperly installed. If the brake drags it could be one of several thing in isolation or in combination with a sticky SMC. It probably all needs looking over. Reginald provided a great overview.
 
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Mike,
I would lay down on the ground on the left side of your bike and turn the rear tire with your foot, then pivot your SMC and see what happens. Or get a helper to turn the rear tire while you rotate the SMC.
Also your SMC pivot bearing may need to be lubed.
I'm in Coquitlam, let me know if you want to come over and we can bleed your brakes and also check your calipers and SMC for binding.
I just bled my brakes over the weekend with amazing results.
 
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Now you may have been able to apply rear brakes only, but even then with linked brakes (rear pedal activates front low piston) it could be enough to grab and depress the SMC a bit.
FWIW, the rear pedal actuates the center pistons on both front calipers. IIRC, the lower bleed screw is for the center pistons. This is very significant when it comes time to bleed the brakes, you'll need to confirm what I just wrote with the SM as I sometimes suffer from CRS.

Don
 
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FWIW, the rear pedal actuates the center pistons on both front calipers. IIRC, the lower bleed screw is for the center pistons. This is very significant when it comes time to bleed the brakes, you'll need to confirm what I just wrote with the SM as I sometimes suffer from CRS.

Don
You beat me to the reply....dang big thumbs and small phone...
 
Joined
Jan 8, 2011
Messages
7,066
Location
Arizona
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I just fixed multiple problems with my breaking system, mostly SMC. Some of which the local shop caused and some caused by a bad pivot bearing.
1) The pivot bearing which allows the SMC to move fore and aft failed. The bearing delaminated the chrome and caused burrs in the inner race causing the SMC to lock up. I took the bearing race out, sanded it removing the burrs, cleaned the bearings with brake cleaner, repacked the bearing with EP lithium grease, and reassembled. It rotates smoothly now. I will see how this works over time; may have to replace it.
2) The SMC plunger under the boot had dried up leaving a concrete looking substance under the boot. I cleaned this up with brake fluid and packed the boot with brake caliper grease to prevent water and air from getting under the plunger. The SMC now moves fore and aft about 3mm and stops the rear wheel from spinning when compressed.
3) The brake system was improperly flushed and bled. I followed the instructions on this site, IGOFAR, and SM for bleeding. The proportional control valve bled black fluid because the shop did not flush it as they should have. This is the line leading to the SMC.
4) The shop used EBC brake pads which are thicker than the OEM brakes and pushed the caliper pistons almost flush to the caliper. I haven't replaced them yet due to budget; spent it on poor work from shop. They rub more than they should. The rotors are not hot while riding, but I like my front wheel to spin more freely when off the ground.
5) I cleaned the brakes according to Joe's article on this site. The pistons weren't bad but the pins had some build up, possibly not releasing the pads properly. I also put a dab of brake grease on the retaining pads of all calipers.

Hope some of this helps.[/QUOTWow Reginald great explanation! Mr. DD also made a good and often overlooked point of the sliding dowels.
 
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I've got 133K on my original SMC and I'm thinking I need to replace this soon. In the meantime, should this thing fail while out on the road. Is there an easy workaround to just get me going again? Steve said he was able to continue to ride by not using his foot pedal. Will this work with every SMC fail?
 

dduelin

Tune my heart to sing Thy grace
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I've got 133K on my original SMC and I'm thinking I need to replace this soon. In the meantime, should this thing fail while out on the road. Is there an easy workaround to just get me going again? Steve said he was able to continue to ride by not using his foot pedal. Will this work with every SMC fail?
In the past we have seen owners jam a wedge like a rubber door stop between the caliper assembly and the fork leg which prevents the SMC from being compressed. This would still allow normal use of both brake lever and pedal but the linked action would be disabled. On a trip it might be hard to find a suitable wedge nearby....:(
 

Throttlejockey

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Do you ride in rain a lot or wash the bike frequently? I packed silicone grease under my boot when I replaced mine. I also tape off the SMC when I wash the bike.
 
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