SMC not moving at all when braking

sethhuner

Joined
May 16, 2025
Messages
13
Age
38
Location
Kentucky
Bike
ST1300
Hi again.

So I replaced the smc with a new one and bled the system, and now the smc doesn't move except if you push it forward by hand. Meaning it doesn't move at all when pushing the front brake lever and it doesn't move at all when pushing the rear brake pedal.

Both the lever and the pedal are pretty firm when grabbing the lever/pushing foot down on pedal.

I just want to ask - what does it mean in this situation where the smc doesn't move? The rear wheel stops spinning when I push the front lever and when I push the foot brake pedal. However on the front it only stops spinning when I push the front brake lever.

Thanks in advance for any advice/tips/help

Also the rear brake disc heated up a lot when i took it for a ride, more than twice as hot as the front brake discs...which were only warm. The rear brake disc was almost too hot to touch.

I do get at least 1.5 turns from the rear wheel when I spin it by hand in neutral...

is my bike good to go now?
 
The SMC is moved by the motion of the front brake disc/caliper, not by application of the brake lever or pedal.
Put the bike on the center stand. lift the front wheel off the ground (floor jack) spin the front wheel and then apply the brake lever. You will see the SMC move a very slight amount which is all it does.
 
You should also be able to move the SMC by hand. Its piston will move a very small amount. Maybe a mm or even less.
If you want to test its function, put the bike on the centerstand, spin the back wheel and use your hand to "rotate: the SMC. If it is working properly, the back wheel will stop.
 
As others have said, the SMC won't move unless the bike, or more specifically the front wheel, is rotating when the brakes are applied.

I attached a GoPro to my left fork and recorded the SMC in action.

SMC in action! | ST1300-Brakes

Among the things I didn't think would be an issue on my pristine 2012 ST1300A with 3,600 miles on it when I bought it in the summer of 2020 would be a non-functional SMC the likes of which seemed limited to those bikes with more years and certainly more miles than my new-to-me bike had...
www.st-owners.com www.st-owners.com
 
sethhuner said:
I only see it when its parked and i push the lever/pedal and it doesn't move.
Click to expand...
The easy way to test the SMC is put the bike on the bike stand, lay on your left side on the left side of the bike with your head near the SMC.
Now try to turn the rear wheel with your foot of choice and get the rear wheel rotating.
Now once you get it slightly rotating apply the SMC by pushing it forward with your hand.
You should feel the rear brake stop the rear tire from rotating.
This will tell you that you have a working SMC.
Once you release the SMC with your hand, the rear wheel should rotate freely.
 

You should be able to push the SMC with your foot while standing. There should be some resistance while pushing it and it will move back to its at rest position when the foot pressure is released. As mentioned the travel distance is very small but felt.

If it clunks when pushed with little to no resistance and falls back to rest and sounds like a door knocker... Houston you have a problem.

Great video!
 
I changed the brake fluid today. I used a brake bleeder, and it worked perfectly. I have a very good brake feel at both the front and rear, so I'm sure there's no more air in the system.

However, I have a question: In steps 4, 5, 6, and 7, does the front secondary brake cylinder still need to be 15 degrees off-center, or can it be reattached to the left fork leg after step 3?

Greetings from Germany, Holm
 
The front left SMC bracket needs to be tilted until you have all of the air out of the system - steps 3-7. Steps 1 and 2 - ie the front upper bleed point for both front calipers - it is probably easier to get those bled and bubble free before embarking on steps 3-7. That is because you can leave the pads in and get a good feel at the lever for how well the two outer pistons are biting. Then focus on steps 3 to 7. The two stages are completely separate, hydraulically.

The 0-15 degree tilt is needed because the outlet port of the SMC needs to be above the inlet port so that bubbles can float up and out. There is no significant reverse flow possible through the SMC. If the SMC is in its normal position, the bubbles will float towards the bracket, and no amount of bleeding and pumping is going to shift them. Pumping the SMC with the rear outer bleed valve or the PCV bleed valve is an important part of the bleeding process. It ensures that any bubbles and brackish fluid within the SMC is purged through the outlet port.

I normally put a pieces of wood into the left front caliper for bleeding the rear system steps 3-7, to stop the centre piston from being ejected.

When yua re nearly done - then leave the brake pedal pressed down or tied down, and the brake lever tied back to the bars. This put pressure in the tubes which squeezes the air bubbles so that they are fraction of the size they were before. With little surface area, they are less able to stay attached to the walls of the brake hoses, so they float up to the nearest high spot. The bubbles merge together to make bigger bubbles which are so much easier to get rid of in the morning.

This link may be of use if you haven't already seen it. Download the pdf in the first post.
 
Thanks John!

Before changing the brake fluid, I remove the brake pads, clean the pistons, reinstall the brake pads, and push them apart as far as they will go, wedging a piece of wood in between. This also removes the old brake fluid that's behind the brake piston.

I bled the brakes again this morning using your method, and everything was fine.

holm from Germany
 
The way I tested my SMC was to let the bike idle in 1st gear on the center stand. Then squeeze the left caliper/SMC and left fork leg together so as to move the assembly forward manually. If it’s working normally you’ll hear the engine slow down as braking pressure is applied to the rear. Be careful not to pull the left caliper/SMC forward very hard lest you accidentally pull the bike off the center stand, hence my use of the word squeeze.
 
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