- Joined
- Mar 18, 2006
- Messages
- 2,994
- Age
- 70
- Location
- Ilkley, W Yorkshire, UK
- Bike
- 2013 ST1300 A9
- 2024 Miles
- 000679
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.
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.
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