SMC slack adjustment

dduelin

Tune my heart to sing Thy grace
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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?
If the pedal hardens after a pump or two that indicates air in the system. If you approach a stop with both front and rear brakes applied approximately equally and you ease off the front only, leaving the rear applied, and the pedal "drops" a little to remain applying rear brake that indicates air in one of the rear circuits.
 

Kevcules

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If the pedal hardens after a pump or two that indicates air in the system. If you approach a stop with both front and rear brakes applied approximately equally and you ease off the front only, leaving the rear applied, and the pedal "drops" a little to remain applying rear brake that indicates air in one of the rear circuits.
Thanks Dduelin....I may have to check that before, or during my first riding season with it soon.
 
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Congratulations, I think you may have the symptoms of a typical Honda Secondary Master Cylinder (SMC) Fail.

How easily should the piston compress?
At rest the SMC piston should be fully retracted, short of disassembling the SMC, the best indicator is that there is NO play of the SMC Push Rod along the axis of movement. From this position the SMC piston should move forward (into the SMC cylinder) a few millimeters before meeting firm resistance.

I pulled the caliper, removed the circle clip and removed the part that contacts the piston. The outer bore looks immaculate.
Unfortunately, unless the SMC piston is completely removed from the SMC cylinder, the bore cannot be fully inspected, the good news is that the limited bore surface that has been exposed is not corroded.

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.
Exactly. The foot brake (Rear Master Cylinder) will exert force on the SMC piston by way of the Recuperation Port, push a bit harder, and you'll force the SMC piston completely out of the SMC cylinder. The SMC piston should move forward into the SMC cylinder with minimal resistance until the SMC piston passes (thus blocking) the Compensation Port, and applies force to the rear caliper.

I figure that's probably normal since it's a hydraulic circuit.
Difficult to tell without a full inspection of the SMC cylinder bore.

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.
Exactly. When the SMC was reassembled, the SMC piston (probably) was fully retracted. Applying the foot brake would not cause the SMC piston to move from it's fully retracted position. Applying the SMC by way of physically moving the unit causes the SMC piston to move as intended, that the SMC piston does not return to the fully retracted position when physical force is removed is a pretty good indication that the SMC cylinder bore may have become corroded. The SMC piston return spring is insufficient to overcome the increase in piston to cylinder resistance due to corrosion.

Might have to pull the piston and clean it up?
Exactly. As others have pointed out, brakes are an important component in the safe operation of the motorcycle. Depending on your mechanical skills, this maybe a task that you are comfortable with, or you may want to have a specialist do the work.

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?
Spongy brake pedal/lever is the usual indication of air captured in a hydraulic brake system. Something in the SMC is causing the SMC piston to 'stick' within the SMC cylinder, the most likely culprit is dirt contamination and corrosion of the SMC cylinder bore.


Here are a few links that maybe of interest:

Drawing - Linked Brake System
Drawing - Secondary Master Cylinder
Thread - Is This The Dreaded SMC Failure?
Thread - How to clean the SMC inside properly? Please help.
Thread - Come on Honda, I expect more from your engineers!
 
OP
OP
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Thanks Annas dad. I pulled the AMC cylinder today and it is absolutely perfect. Not a speck of dirt, corrosion, anything. I reassembled and bled out a ton of air (spent twice as long as it took when I flushed the system 2 months ago). Haven't been able to take her out for a test but preliminary results look promising.
 
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Not a speck of dirt, corrosion, anything.
That's great!

Make sure the two pivot areas are well lubricated, and you've addressed all the mechanical binding areas of the SMC.

I've found that the hydraulic brake systems on both the ST1300 and the GL1800 are the most frustrating brake systems I've had to bleed air from. I also find it amazing how frequently these systems require bleeding of air, every automobile I've had will go a lifetime without requiring annual bleeding.
 
OP
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Got her out on the road today for a few miles and it looks like it was air related. Having just flushed out the brake system a couple of months ago, I'm guessing I didn't spend enough time bleeding the PVC and didn't have the SMC tilted enough. This time I made damn sure the SMC was tilted enough by hanging it from the handle bars with a bungee. Had it almost vertical with the outbound like e on top. After about twice as long as I spent bleeding the PVC last time I got about a dozen bubbles out. Kept up for a bit just to be sure, then bled the rest of the system.

I've worked on cars and motorcycles since I was about 8 and have never had one that took that long to bleed out.

There is still a noticable (albeit much quieter and less metallic) noise when I activate the rear at low speed but I'm assuming that is normal based on the way it functions. Manually, there is very little movement of the caliper before it starts compressing the piston and the feel is clearly better.

Strange thing is, the pedal feel is the same. I am used to air in line lines giving a very spongy feel. Didn't have that before. Felt (and feels) very firm. Oh well. Chalk it up to another learning experience on a new machine. She needs a little more attention to the brakes but is easily worth the extra time.
 
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One thing I'd add about the SMC, it is relatively easy to push air bubbles down a brake pipe, but the SMC bore is considerably wider , that's why it must be tilted to the correct angle, if there is air trapped at a high point no amount of bleeding will clear it.
FWIW as well, I had an issue that sounded like the OP issue, The SMC was sticking in a little and this meant the rear always felt like it had a bit of slack for the first press, what happens its the first part of fluid movement from the rear pedal actually pushes the SMC piston out prior to the brake being applied.
 
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I also wonder if Honda have binned the SMC now because of these issues, on my 13 VFR1200F there is still a link from the rear to a set of front pads but no link to the rear from the front brake.
 
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IMHO it still sounds like something is not quite right in your brake system.
Your brakes should not be making ANY kind of noise that you describe when working properly.
With no disrespect, despite your mechanical experience since eight, perhaps you should ask for some assistance to someone near you that has experience in bleeding the system on your ST1300.
There are a lot of places for air to hide and be missed that you may not be familiar with, and simply hanging the smc won't insure success.
If your still hearing noise something is not right.
I have removed air from bikes straight off the showroom floor, that the owners swore were the best brakes ever.
.02
 
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This link is not to old. I would like to ask just one question. I have bled my system 3 times with no success. Is there anything beside the SMC that will make the SMC incredibly tight. Could air in the wrong place cause it?
 
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