Article [13] ST1300 - Brake Maintenance - Replacing the Secondary Master Cylinder (SMC)

[jfheath]

The usual format from me - Description, some useful photos and a few tips and tricks to help you to avoid the normal 'Doh!' moments -

• Like emptying all of the fluid out before realising that you wanted tomove the pistons out as far as possible;
• Like reftting everything before realising that you cannot get your torque wrench in to fasten the last bolt;
• Like checking that the new SMC has been greased under the boot and then discovering that you can't replace it properly.

Anyway - there is a sequence, but it is not intended that you follow it. Just read it through before tackling the job. You may find the information useful.

Feel free to download, save or print this.

And if I mention @Mellow he may put it into the articles section !

 

[minhtri2002]

It’s awesome for someone who doesn’t know how to replace it. I believe any Honda dealership now closing soon. Ha ha ha.
You’re the best. Thanks!

 

[thekaz]

truly hate the "subject" but love the details and pictures

 

[Chris09]

and a great short description of both the SMC and proportional valve operation - thanks

 

[STRider]

Thank you John, but as usual my timing was impeccable!

My garage, July 4, 2022, mid-SMC replacement.



All went well, and I've completed a successful teardown of the defective SMC in preparation for a rebuild. Will report on that work in a different thread later.

 

[jfheath]

Brilliant @ST-Rider I'll keep an eye open for that - see if I missed anything !

 

[Burttj03]

Brilliant write up, @jfheath! Thank you so much for all of the effort that you put into these articles--they are truly helpful, and very much appreciated! Your commitment to educating others on how to properly carry out maintenance procedures on this bike is incredibly inspiring! I can only imagine the effort that goes into creating these articles, and I just want to personally thank you for all the knowledge that you've provided to us on this forum! You, @Igofar, and so many other members on here have been invaluable resources for myself and countless others! Sincerely, thank you all!

 

[craigsif]

Hi Ive got a air in system problem, bled system several times following the files, but rear pedal keeps going soft, have replaced all piston seals,master cylinder,SMCcylinder done, gone through bleeds, around 7 times, rear brake pedal goes firm, 5 minutes later soft, ive noticed that when Ive applied front brake, rear pedal very hard. again 5 minutes later soft. Have left over night with the brake levers under pressure, check im the morning system fine, then rear brake soft , Ive just gone through the whole procedure again and left brakes under pressure, but I am at a loss what next to do. any advice appreciated, as she is due for MOT tomorrow,

 

[woodybelle]

Call Igofar. Seriously

 

[jfheath]

When you say 'SMC cylinder done' - what exactly do you mean ?

You dont have much time, so I'll throw suggestions.

1. Your SMC isn't sealing. and is not able to return properly. eg due to an oval bore.
The SMC is moved when you brake, but it cannot return by itself. Applying the brake pedal puts pressure in the line behind and helps move the piston back, resulting in the soft pedal feel. Typically, you can tell this is happening if you pump the pedal and it stays firm for a while.

This will not last forever. At some point it will fail drastically, if the piston cannot clear the compensation port. If the piston has stopped sealing due to an oval bore, then it will probably stay like this forever. I have seen this situation before.

2. Did you fit a service kit ? Did you ensure that the seals went in properly - flared open end first ?

3. You still have air in the rear system. And I can probably explain where it may be hiding:

The rear brake line goes from the rear master cylinder to the rear centre piston. That is easy to bleed, so I am guessing that it is not hiding in that line.

Another line goes from the rear master cylinder, forward, up a flexible hose and down the front right fork leg. It splits at the top of the mudguard, one line going to the front right centre piston, the other across the mudguard to the front left centre piston.

The front left centre piston has a very short hose to the inlet of the SMC.

Fluid then passes through the SMC, but it can only do this if the SMC is relaxed.

You said that when you apply the front brake, the pedal is firm. Since the front brake line and rear brake line are not connected, so this isn't possible if the bike is stationary, I will assume that this happens when you are riding. The front braking operates the SMC when the bike is moving and this effectively prevents the fluid from the rear brake passing through the SMC.

From this, it seems logical that there is no air in the line from the pedal to the front left centre piston bleed valve.

So the air is likely to be from there onwards. ie up the left fork leg, back across the mudguard, through the proportional control valve (PCV), past the bleed valve near the PCV on the right hand side under the tank near the ignition coil, and then down to the two outer pistons in the rear caliper.

The most likely place for the air is in the flexible hose which forms an 'n' shaped loop behind the upper fork legs. Get a vacuum pump on the Pcv bleed valve and let it suck fluid while you flex and tap that hose. ( there are two hoses, you need to flex the correct one).


I think that It is either going to be a VERY long night, or you are going to miss your MOT !

 

[Igofar]

Thanks John

 

[jfheath]

He's in the Uk, Larry. I'm sorry, I don't have time to call - we have stuff to do and places to be in the next few days.

 

[Igofar]

No I meant thanks John for posting your comments to try and help.
He reached out to me, and while I can’t call the UK from my phone, I will try and message him later tonight with some instructions and things to try.
You have a nice day and relax, I’ve got your back

 

[craigsif]

When you say 'SMC cylinder done' - what exactly do you mean ?

You dont have much time, so I'll throw suggestions.

1. Your SMC isn't sealing. and is not able to return properly. eg due to an oval bore.
The SMC is moved when you brake, but it cannot return by itself. Applying the brake pedal puts pressure in the line behind and helps move the piston back, resulting in the soft pedal feel. Typically, you can tell this is happening if you pump the pedal and it stays firm for a while.

This will not last forever. At some point it will fail drastically, if the piston cannot clear the compensation port. If the piston has stopped sealing due to an oval bore, then it will probably stay like this forever. I have seen this situation before.

2. Did you fit a service kit ? Did you ensure that the seals went in properly - flared open end first ?

3. You still have air in the rear system. And I can probably explain where it may be hiding:

The rear brake line goes from the rear master cylinder to the rear centre piston. That is easy to bleed, so I am guessing that it is not hiding in that line.

Another line goes from the rear master cylinder, forward, up a flexible hose and down the front right fork leg. It splits at the top of the mudguard, one line going to the front right centre piston, the other across the mudguard to the front left centre piston.

The front left centre piston has a very short hose to the inlet of the SMC.

Fluid then passes through the SMC, but it can only do this if the SMC is relaxed.

You said that when you apply the front brake, the pedal is firm. Since the front brake line and rear brake line are not connected, so this isn't possible if the bike is stationary, I will assume that this happens when you are riding. The front braking operates the SMC when the bike is moving and this effectively prevents the fluid from the rear brake passing through the SMC.

From this, it seems logical that there is no air in the line from the pedal to the front left centre piston bleed valve.

So the air is likely to be from there onwards. ie up the left fork leg, back across the mudguard, through the proportional control valve (PCV), past the bleed valve near the PCV on the right hand side under the tank near the ignition coil, and then down to the two outer pistons in the rear caliper.

The most likely place for the air is in the flexible hose which forms an 'n' shaped loop behind the upper fork legs. Get a vacuum pump on the Pcv bleed valve and let it suck fluid while you flex and tap that hose. ( there are two hoses, you need to flex the correct one).


I think that It is either going to be a VERY long night, or you are going to miss your MOT !

Thanks,

 

[craigsif]

Thanks, bike passed mot brake held firm, but i think looking at your reply its the front SMC?
As the brake pedal goes hard when i pull on the front brake when stationery so if that is failing then as you said fluid will push pass?? Then would it pressurise the rear brake??? Does that make sense i havent done the front smc!! But i do have a replacement kit in its way,

 

[jfheath]

Glad you passed the MOT!

I've confused you. Sorry. You need to have a good picture in your head about how the brakes are connected. I'll put a link at the bottom of this post - it shows a series of diagrams to explain how fluid reaches each of the brakes and bleed points.

It sounds very much like the SMC bore isn't sealing - possibly been knocked out of shape. The description you gave is pretty much the same as the situation a friend had. He fitted a service kit and it continued to do the same thing. A new SMC fixed it. He sent me his old one, which I investigated and then took a hacksaw to expose the full length of the bore. There are photos of it somewhere on here.

The rear brake going hard when the front brake is applied when the bike is stationary ? I don't see how that can happen. The front brake lever operates the two outer pistons in the front caliper. Nothing else. I need to think about this

I can see how the rear pedal may dip slightly when the front brake is applied: If the rear pedal is pressed it operates the front and rear centre pistons. When the front lever is applied, the two outer pistons at the front operate. The two outer pistons have a greater combined braking for e than the sungle centre piston, so they squeeze the pads harder onto the disc - leaving the centre puston applying little pressure at all, so with the same pressure still on the brake pedal, the lever can push the centre piston out a little more - so the prdal dips a tiny bit.


Pretty pictures. Download the pdf in post"1

Article [13] - ST1300 Maintenance - Getting all the air out of the brake system | ST1300 Articles

A detailed look at the process of getting the air out of the Honda ST1300 brake lines. A sequence of diagrams of the brake circuits, colour coded to show which parts of the system are being accessed when each bleed valve is opened and explanations of where the trouble spots are. These are just...

www.st-owners.com

 

[craigsif]

When you say 'SMC cylinder done' - what exactly do you mean ?

You dont have much time, so I'll throw suggestions.

1. Your SMC isn't sealing. and is not able to return properly. eg due to an oval bore.
The SMC is moved when you brake, but it cannot return by itself. Applying the brake pedal puts pressure in the line behind and helps move the piston back, resulting in the soft pedal feel. Typically, you can tell this is happening if you pump the pedal and it stays firm for a while.

This will not last forever. At some point it will fail drastically, if the piston cannot clear the compensation port. If the piston has stopped sealing due to an oval bore, then it will probably stay like this forever. I have seen this situation before.

2. Did you fit a service kit ? Did you ensure that the seals went in properly - flared open end first ?

3. You still have air in the rear system. And I can probably explain where it may be hiding:

The rear brake line goes from the rear master cylinder to the rear centre piston. That is easy to bleed, so I am guessing that it is not hiding in that line.

Another line goes from the rear master cylinder, forward, up a flexible hose and down the front right fork leg. It splits at the top of the mudguard, one line going to the front right centre piston, the other across the mudguard to the front left centre piston.

The front left centre piston has a very short hose to the inlet of the SMC.

Fluid then passes through the SMC, but it can only do this if the SMC is relaxed.

You said that when you apply the front brake, the pedal is firm. Since the front brake line and rear brake line are not connected, so this isn't possible if the bike is stationary, I will assume that this happens when you are riding. The front braking operates the SMC when the bike is moving and this effectively prevents the fluid from the rear brake passing through the SMC.

From this, it seems logical that there is no air in the line from the pedal to the front left centre piston bleed valve.

So the air is likely to be from there onwards. ie up the left fork leg, back across the mudguard, through the proportional control valve (PCV), past the bleed valve near the PCV on the right hand side under the tank near the ignition coil, and then down to the two outer pistons in the rear caliper.

The most likely place for the air is in the flexible hose which forms an 'n' shaped loop behind the upper fork legs. Get a vacuum pump on the Pcv bleed valve and let it suck fluid while you flex and tap that hose. ( there are two hoses, you need to flex the correct one).


I think that It is either going to be a VERY long night, or you are going to miss your MOT !

Hi John before I confuse everyone Doh, I fitted a second hand one as my original was causing the back brake to lock on, after reading what you kindly explain, on how it works and the possible cause, I went and checked my original SMC and found the piston was stuck, removed this cleaned and checked the bore, found no marks or pits , so have rebuilt it using a new SMC kit, flushed it including the small filter! Tomorrow I will take the other from the bike and refit this rebuilt one, and go from there. Once again thanks to both you and Larry, for your fantastic advice. Craig

 

[Esti1300]

Terrific write up John (@jfheath), thanks for adding some clarity to an often cloudy subject.

That being said, I do have a couple of questions as I prepare to replace my SMC. Just a straight replacement. Out with the bad, in with good. New parts are on hand, tools readily available to be mis-used and armed with the knowledge I have gained from several reading sessions on the subject.

With no experience removing any type of caliper let alone one that has a SMC attached to it, could I not just pinch off the 1, 2, and 3 brake hoses to limit the volume of fluid lost? What would I use to pinch off a brake line? Threefold reason for my question:
1) Having never done this before, I don’t know what kind of flow rate I will face by opening up the system
2) Would this not prevent too much air from entering the system?
3) I don’t work fast so this would buy me some time and awkward fumbling with plastic bags and zip ties.

I do understand that regardless, a full bleed of the brake system will be necessary once the replacement parts are in place. Guess I’m just trying to visualize how this will go.

 

[jfheath]

You're welcome. Sitting here in a hotel lounge still grinning from a day spent breaking in a brand new rear shock absorber, and now savouring a favourite whisky - Glenfarclas.

In reply to @Esti1300 - I never pinch the brake lines. You have put on a new SMC. It comes with air pre-installed. you've removed the banjo bolts and let in air. Who knows where it goes ? Well, I do. It goes up, and fluid drops out. You can limit the loss by holding the end up, but frankly, once air is in the system, you are going to have to bleed the entire circuit.

That's not quite right. The front outers are not affected. The chances are that the rear centre isn't affected.
So I would start pushing fluid to the front centre bleed valve, then through the tilted smc and bleed at the pcv valve.
Then I'd go round the rear circuit in the correct order, making sure to flex and tap lines. You will have to do it anyway if you want to get rid of the air. There isn't a short cut really.

If you don't have a vacuum tube to help, then remember the trick of pumping the pedal a few times holding the pedal down and then opening the valve - closing the valve before the flow stops.

 

[Esti1300]

Well deserved! Enjoy.