Right Quick ST1100 response.
It would be really helpful to know which ST1100 model you have. I am assuming 1999 or 2000 - 3 pistons per caliper.
I believe that the Combined braking system (CBS or LBS) which has the SMC was only available with the ABS model. Honda's documentation history of models suggests that this was introduced in 1996, but the fiches for the UK do not include a secondary master cylinder until 1998. The ST1100 AW. The SMC is mounted on the front left fork leg, but it is mounted vertically, with the open end pointing downwards - very much like the rear master cylinder.
The SMC on the 1300 faces upwards and is subject to damage to the bore if not serviced correctly.
The SMC on the ST1100A is hidden behind a grey plastic shroud and is attached to the the fork leg with two curved metal plates and an 'Ace of Clubs' linkage. This connects together the fork leg, the SMC plunger and the top of the caliper bracket. The action of the front brake grabbing the disk moves the caliper forward and up - which pushes the plunger of the secondary master cylinder and applies the rear brakes. The linkage is subject to rust, decay and crud chucked up from the road surface, and because it is hidden behind that plastic shround on the mudguard, the decay is not visible at a casual glance. In Ireland you probably get more of this than we do in West Yorkshire.
A couple of the holes in this linkage have needle roller bearings in them (see the green part on the right, and this entire mechanism can seize up. The braking force to operate the SMC is massive. But it has only the spring inside the cyclinder to return it. When it gets stuck, the piston will not return, the main seal in the SMC will not clear the pressure relief compensation port, and it will keep the pressure on the rear brake.
Would I service a rear master cyclinder with a service kit. Yes I would.
I have never had the ST1100 SMC apart - but I have owned an ST1100 ABS/CBS/TCS bike with the SMC arrangement as shown in the picture on the right - and I have performed this clean-up operation. Would I service that with a service kit ? I don't know - but I would have a good look at it and consider it. Unlike the ST1300, the pressure end is at the top and it is sealed. The end that may suffer from ingress of water at the boot is at the bottom, and the operation is very similar to the rear master cylinder operated by the brake pedal. But if I bought a service kit, I'd buy the OEM one. I have had a situation where a friend came round for help with locking front brake, and it turned out that his cheap replacement service kit had a plunger that was a tiny fraction too long - so his lever was always applying a bit of front brake. It pushed the piston main seal beyond the tiny compensation port, so having applied the brakes, they could not release pressure and his front wheel was locked solid.
One thing is for certain - the length of that plunger is critical. Yet it seems to be screwed on and have a lock nut. I don't know how easy it is to get at the circlip with that in place, so there will be a very strong temptation to remove it. And that is where the danger lies. However, the service manual does not make a big issue of this. There are torque values for two nuts - so I guess the first nut just tightens agains the end of the thread. However, to be sure, I would still measure it before taking it apart - as described below.
If you are determined to 'have a go' then you must extend the SMC plunger as far as possible and find a way of measuring - very accurately - the length. It doesn't matter which reference points you use, as long as you can measure it again from the same points when you put it back together. It has to be identical. So you need an accurate caliper with a vernier scale and you need to know how to read the vernier scale. Something that will give you a measurement accurate to 0.01mm. The cheaper electronic ones are not usually accurate enough.
The amount of movement at the SMC is really very small. Get this wrong and you risk a rear wheel lockup - which could be fatal.
But the good news is - if this has been serviced well (ie fluids changed AT LEAST every two years) then the problem is more likely to be with the linkage, not with the SMC. So it is certainly worth undoing the bolts, inspecting the bearings, cleaning them up. and reassembling.
When I did mine the rust on the metal plates was preventing the assembly form moving. I removed them, cleaned them up with a wire brush and repainted them. The bearing were not turning at all = jammed solid with muck - but no sign of rust. The inner sleeves cleaned up nice and shiny with a bit of paraffin. I removed and reused the seals, and the bearings I left to soak in parrafin and later scrubbed in situ with a small 'tooth brush'. They became loose and rolled freely in the residual parrafin. I then washed them with water and detergent - and they were running freely. Cleaned with rag on a stick which I inserted back and forth and turned - then rinsed with WD-40 to dispel any residual water and then cleaned with clean cloth. Finally packed with ordinary bearing grease, and inserted the inner sleeve to check how well they rolled. Perfect. I should have replaced the seals, but there was never any sign of grease escaping and I checked the movement of that linkage whenever I had the wheel out.
None of this explains why the
front wheel is dragging. But there is still the correct procedure to follow when installing the front wheel.
The issue is that the axle isn't just held in place. When the axle bolt is tightened at the right hand end it actually pulls together all of the components that make up the hub. If the fork pinch bolts are tight, it cannot draw the axle across, so the wheel hub components are not clamped together. The spacers the bearings, the bearing spacer - and they all get clamped and locked solid against the right hand fork leg. I cannot remember whether the speedo bearing is part of this. Probably.
It is important that the right hand pinch bolt is not tight when the axle bolt is torqued. If necessary tighten the left hand pinch bolts to stop the axle from turning - but dont do this until the axle has been pushed all the way through as far as it will go with hand pressure. Once the main axle bolt is properly torqued, undo the left side pinch bolts. It is usually a good idea to bounce the forks a few times, and then the axle bolts at the bottom of the fork legs can be tightened after checking the clearance for the left hand brake caliper against the rotating brake disc. If the clearance isn't at least 0.7mm - both sides of the left hand disc, then undo the left hand fork leg and move the fork slightly along the axle so that the clearance is achieved.
nb Left / Right refer to the side of the bike as if you were sitting on it. Left is the clutch lever / SMC side. Right is the brake lever / Main axle bolt side.
If it isn't done like this then the left caliper may be slightly out of position in relation to the brake disc and (Particularly new) pads will be held permanently aginst the disc surface - which will cause front brake drag, which will cause the SMC to operate which will cause rear brake drag.
And all of the information that is contained in
https://www.st-owners.com/forums/threads/st1300-brake-maintenance-avoiding-the-pitfalls.135125/
is relevant to the ST1100 Pan Europeans that have 3 pistons per caliper.
