Article [13] ST1300 - Brake Maintenance - SMC Boot Fitting - A Cause of Failure ?

[jfheath]

The rubber boot on the ST1300 Secondary Master Cylinder is designed to keep water out. Unfortunately, since you cannot see what you are doing when fitting the rubber boot, it is entirely possible that casual maintenance can result in allowing water to be let in.

I made reference to this in the article ST1300 Brakes - Avoiding the Pitfalls - but someone just checking the bike over would probably not wish to refer to that document - and another member sent me a PM to ask about the use of grease. I thought it was worth making a new thread.

The message is simple. Make sure the relevant components are properly lubricated and ensure that the rubber boot is properly seated.

What follows is just explanation and photographs.

The manual calls for silicone grease to be applied behind the boot area of the SMC plunger and to the contact area between the push rod and the piston.
I have posted these photos before, but this is what those areas will look like if silicone grease is not applied. This was a brand new service kit 2 years previously.



Below is a cross section of how a service kit is fitted into the SMC bore.

The SMC is from an 04 ST1300, so it has a drain hole rather than a drain channel of the later models. Nevertheless, these notes apply to all versions of SMC.




Note that the Brim of the top hat is pressing firmly against the sides of the opening of the caliper bore. The rest of the top-hat boot does not touch the sides. In fact, if you look at the top half of the cutaway, you will notice that there is an angled step. This goes all of the way around the bore, but it is less easy to see at the bottom of the cutaway, because of the drain hole. The brim of the top hat rubber boot must be pushed up that step, all the way round, in order to form a proper seal. The spring inside the brim is strong, and it takes a small amount of effort, but you can feel the point when the spring yields to press against the narrower part of the bore.

In order to press it into position, you need something to push onto the brim, down the side of the top hat. Something without sharp edges that will not damage the rubber boot itself. If the bracket has been removed (which Honda advise against), then something like an 18mm 12 point socket will slip over the top hat, and seat the sprung brim easily. A little silicon grease on the outside of the top hat will help the socket slip off without pulling the boot back again. I must admit to separating the bracket in order to gain better access to the circlip, but I measure the length with a vernier scale caliper first and make sure it is exactly the same when I replace the bracket. Honda are quite clear that you should not do this.



By 'exactly' I mean just that. The SMC moves only a little over 1mm to apply the rear brake. An error of 0.1mm is a massive 10% change compared to that movement. If you need to do this, then get a vernier caliper and learn how to read it to .01mm. Beware digital calipers. Check out their declared accuracy. They might show a nice digital readout, but there's a good chance they are not as accurate as a vernier scale. I just checked the best rated model on Amazon - that has a declared accuracy of 0.05mm. That us still 5% of the movement of the SMC pushrod.

Having told you not to separate the two parts, elsewhere in the Honda Workshop Manual it gives a torque of 18Nm, and adds a 'Note 2' - apply a thread locking agent (General Information tables / Hydraulic Brakes). Please check this information for yourself before using it. Mine is an old manual and has plenty of mistakes in it - and the SMC has changed over the years.

The point is - that brim MUST sit on the reduced diameter 'step' and behind the drain hole. The drain hole is there to prevent the pooling of water in the opening to the SMC, not to allow water behind the boot to drain away. If it is lubricated and correctly seated, water should not get behind the boot.

For 2008 models and later, there is no drain hole. Instead, there is a channel, as shown in the photo below. It is easier to see whether or not the brim of the rubber boot is seated behind the point where that channel starts.

 

[Kevcules]

Excellent explanation!

 

[ST1100Y]

Seems that it is a mandatory once a year inspection/maintenance item...
(as with the space underneath the boots of clutch and brake master...)

 

[bplandis]

On the pre 08 any thoughts on using a dremel to add the "drain channel" Did not spend much time researching but may be a better option that the drain hole which could build up crud and not drain well.

Could cut channel in when boot is removed for inspection or service.

 

[jfheath]

Isn't it less effort to clear the drain hole ? The hole is not a bad design, its just that if you don't clear it, water cant escape. Just use a blunt probe so as not to puncture the rubber if it has moved.

My first ST1300 had the drain hole. I rode through winters - uk roads have a lot if grit and salt on them in winter, and I live in area where sheep, cattle, farm vehicles are all daily engaged in the important yorkshire activity of moving stuff that should be in the fields, onto the tarmac.

I never found that my drain hole became clogged.

 

[woodybelle]

John, how do you remove the boot? Do you have to use a pick tool or can you just pull it out with your fingers? I have tried to get mine out to inspect behind the boot but it did not want to come out easily and I did not want to damage it so I have never had it out.

 

[jfheath]

Don't use a probe - and in fact, if the SMC is functioning properly, I wouldn't be tempted to shift the boot at all. If the seal is good, all will be ok, and you will have disturbed it and have to put it back properly - which is very difficult to do with the SMC on the bike, there is no room to work, you cannot see what you are doing and you really need a tool to do it with. I wont do anything with the SMC assembly while it is still on the bike. In fact I have never been successful in putting the boot back on and seating it properly without removing the bracket (which you are not supposed to do) and using a suitable socket to drive the brim into position. With the SMC in place it is extremely difficult to tell whether or not it is fully seated.

If you suspect the SMC may be playing up, then check it out the next time you have a bleed tube connected to the rear outer or the PCV bleed valve. Pump the SMC full strokes and see how it feels and whether it is able to freely move the full distance. If you are concerned, set aside some time to buy new along with crush washers and anything else you need to do to the brakes. The peace of mind is well worth the outlay in money and in time.

 

[jfheath]

John, how do you remove the boot?

Sorry, apart from giving a lecture advising against it, I just realised that didn't actually answer your question.

I pinch the side of the boot between thumb and forefinger and pull in the direction of the shaft. The boot material is quite thick and sturdy, at least when new.

Just make sure you have the time to deal with whatever you find out, and to cope with ensuring the boot is properly seated when you cannot see what you are doing.

 

[Ralf_CT]

Great write-up! When would one reach the point where, from a preventative maintenance point of view, one strips/checks the entire brake/clutch hydraulic system and replaces the seals. Fluid changes are often overlooked, even on dealer-serviced bikes. I must confess, I've never changed the brake fluid on my 2012 PCX150 (although it's on the agenda), and I'm not sure when/if hydraulic fluids on my prospective 2009 ST1300 were done, even though it has a FSH.

 

[jfheath]

Before doing any fluid change, get a feel for how the brake system is operating at the moment.
spin the back wheel, see how much drag there is. There should be very little if any, and it should do maybe a couple if turns with a hefty spin

It doesn't take much effort to push the SMC forward a tad. It should move only 1-2 mm, and when it does it should lock the back wheel. You can check by turning it with your foot. And crucially, when you release the SMC . Then spin the back wheel again. It should be no worse. If it is, then something is stopping the brakes releasing. It might be the calipers, it might be the SMC. Things to check out.

But I wouldn't bother finding out when it was last changed. Change it yourself anyay and do it properly. Theres plenty of stuff. on here to help. I'll put some links in when I get to my pc.

As for stripping the entire system ... pistons, seals, brake lines ?? Anything would be guesswork. See how good the condition is.
I bought a second hand caliper from an 08 a couple of months ago. Before it arrived, I decided it would require new pistons and seals no matter what. Everything seemed. to be brand new, apart from the fact it was covered in brake fluid. The seals and pistons that came out looked perfect, and there was nothing to clean up in the seal grooves. It was in better condition than my 2013 built 09 model, and I have changed the fluid every year. I could tell that it hadn't been ridden much, but it had certainly been serviced. Old brake fluid crystalises.

 

[MarksMotorcycleParts]

Good write up, loads of useful info. Thank you.

I have a mate’s bike in the workshop now with a dragging rear brake, when cleaning the rear caliper, the middle piston could be pushed back in but not the outside two. Very little movement was possible at the SMC. Released some fluid from the rear caliper bleeder to get the outside pistons in.

After a short ride the rear caliper is noticeably dragging and the front left is too but not as badly. 10 minutes later the wheels turn freely and working the SMC by hand causes the rear caliper (and front left) to grip the disc and then release it, as I’d expect. Confused by why it seems ok in the workshop but not on the road – it’s as though the SMC isn’t releasing while riding. Planning to change & bleed the brake fluid and / or replace the SMC seals but reluctant to start randomly swapping parts after that. Any further suggestions would be appreciated. Thank you.

Experienced bike mechanic but this is my first Honda DCBS job, not sure I want any more. Bike is 2003 with ABS, 22500 miles, generally very good condition.

 

[jfheath]

There are a couple of things you mention that might be if concern.

1. If the front left is dragging, that will activate the SMC when the bike is moving. So a problem with the front left caliper needs to be eliminated first.

This may be due to poor piston movement in and out, poor caliper movement (slide pins, pad spring), corroded/ ridged brake pad pin, incorrect retaining clip fitted - the rear clip has a ridge and if fitted to the front will cause the brakes to bind, heat shield from rear fitted on front (front doesn't have a heat shield -there is no room for one when pads are new).

It may also be due to the compensation port in the rear master cylinder not allowing fluid to return to the reservoir, or the rear reservoir being brim full.

Check if you can push in the centre piston of the front left by hand. This will check the line to the rear master cylinder and reservoir.

However, I would suspect that both front brakes would drag if this line was blocked.

2. If the front left caliper is proved to be ok, the. turn your attention to the rear caliper. Check that pistons move freely and that the slide pins, pad spring and pad pin are all as they should be.
If you put a bleed tube on the front left centre piston, try pushing in the rear outer pistons. If you can, then that suggests that the line through the SMC compensation port is clear.

With the front left centre bleed valve closed, you can expect the resistance on the rear outer pistons to be greater. The line is much longer and there are two compensation ports for fluid to go through. See-sawing the two rear outer pistons usually helps, or preventing one from moving out and using both thumbs on the other.

You need two documents to understand these brakes

You will know all about bleeding air, but the diagrams in here show you the layout of the hydraulics in relation to the brake lever/pedal, reservoirs and bleed valves.

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

And much of what is written here about the rear caliper also applies to the front calipers.

Article [13] - ST1300 Maintenance - Brakes- Avoiding the Pitfalls | ST1300 Articles

A close hands-on look at the Honda ST1300 braking system. These are personal observations about the things that can go wrong, why they go wrong and what can be done to keep things working sweetly. Updated April 2020 and to address more recent observations, to include information about the SMC...

www.st-owners.com

There is still the possibility that there is a mechanical issue with the SMC - the bike is old enough for that to be a strong contender for dragging rear brakes. But you said that the front left was dragging and that the rear pads were releasing after the Smc had been activated and released by hand.

So my first suspicion lies with the front left brake.

ps the retaining clip is that chromed plate that sits between the end of the brake pad and the caliper bracket - the opposite end of the pad from the pad pin hole.

Note the orientation of the pad spring. On the 2003 machine it is identical to the one in the rear pad and it possible to fit them the wrong way round. This is illustrated in the Avoiding the Pitfall Articles linked above.

There's more to look into but the above are good starting points given the symptoms that you describe. Again given the age, a brand new SMC is a good investment. Getting a repair kit is often not as effective as you might hope - the SMC is exposed, and some failure is likely due to corrosion in the bore or from damage to the bore after a slight knock - neither of which are fixed by replacing the internal piston, spring and seals.

Note that the design of the front calipers, pads, pad springs, pad pins, caliper bracket and SMC changed signficantly in 2008 and later models. In this regard, they are not 'all the same' !

 

[MarksMotorcycleParts]

John, Thank you so much. I need to re-read all that tomorrow when I'm a bit less tired but already I can see you've given me several new things to look at before spending money on random parts. I'll add more

 

[MarksMotorcycleParts]

After reading the two attached articles, I stripped and cleaned all calipers & pistons again, and found that it was indeed the left front caliper causing the brake to drag on which in turn put the rear brake on. Many thanks for your help.

 

[jfheath]

Problem solved. The solution to the dragging rear brake turned out to be related to the front wheel installation and slight fork misalignment.

 

[Chris09]

Problem solved. The solution to the dragging rear brake turned out to be related to the front wheel installation and slight fork misalignment.

Thanks for the tips, I've copied and pasted this whole discussion for reference on my own [would like to call it slightly, might be wishful thinking] dragging rear caliper assembly; there's a number of points in there I didn't pick up in some of my previous readings by jk, but I'm a useless reader most of the time.
I never noticed any problems while riding, disc temperatures seemed okay [to me, the expert], supposed proper maintenance, low miles and little evidence of exposure to the elements, but on the center stand there's no way that rear wheel will do two revs. the left caliper rocks smoothly, SMC seems to move nicely everything is super clean, so maybe I'll check those initial reservoir levels first, get the front wheel up, and exercise the front hydraulics a few times with the wheel rotating; see if I can get the front left to drag a bit after releasing, see if that might be holding in that SMC a bit. I do go about things ___ backwards, but at the same time, replacement parts are expensive, far away and what's worse, time away, I haven't got myself so far that I won't be able to reassemble and ride, get everything for the next lay up.
In any event will move my little $30.00 infra red thermometer to the tool kit and be gunning those discs and calipers - often.

 

[jfheath]

Before you do that - pop the rear pads out, don't touch the brakes and see how much rotation you get then. It is entirely possible that the bearings are a tad tighter than they should be.
No matter what you do looking at pistons, smc, bleeding etc, the back wheel will never spin more than what you can achieve without the pads in !

 

[Igofar]

Problem solved. The solution to the dragging rear brake turned out to be related to the front wheel installation and slight fork misalignment.

Good Job Mr. Heath
This is another example of why I inspect and correct the front forks on all the bikes that come through the garage!
The forks being out even a little bit, can cause the SMC to have issues etc.
Another issue I've found a lot lately, is folks putting the left front wheel spacer in backwards, only to figure it out after their first attempt to ride it, then correcting the direction of the wheel spacer, WITHOUT inspecting the wheel bearing DUST SEAL, and not noticing that they crushed it flat, and THAT is now causing the wheel to be causing issues.

 

[ad.hom]

not sure I want any more

And there I was, about to bring you my '02 with repeatedly dragging rear brakes, from Ruthin.