OK, I need some help. If the bike is on the center stand (ie. stationary), and the rear brake is applied, how does the SMC come into play in this case? I was always under the understanding that the SMC was actuated when the front caliper (containing the SMC) grabbed the disc and caused the SMC to move slightly. Therefore, a faulty SMC would not cause sticking of the rear wheel after application of the rear (or either) brake without the bike being in motion. Not saying that the SMC does not need attention but how does it cause the issue noted by the OP?
Good question. I think it was this same problem that people were describing years back that made me want to find out exactly how this complicated system worked.
Are you sitting comfortably ?? Then I'll begin.
The fluid from the rear master cylinder pumps fluid to all 3 pistons on the rear caliper when the bike is stationary. There are two banjo bolts bolted to the rear master cylinder each with a hose out. One line goes direct to the rear caliper centre piston. Forget about that for now. The other line goes to the front centre pistons. The right hand side goes through a delay valve, which will casue the right hand centre piston to operate only after the preesure at the pedal is sufficent. (To prevent excessive dive on the front). The line to the left hand caliper goes to a Banjo bolt on the SMC and this has an extra hose attached which feeds the front left centre piston - which is operated immediately.
With me so far ? Ok here is the answer to your question.
The pressure from the rear brake pedal feeds fluid into the inlet port of the SMC. When the SMC is not operated - by the action of the SMC bracket rocking forward when the front brake grabs the brake disc when it is rotating - then the piston in the SMC should be fully at rest. The pressure from the operation of the brake pedal pushes fluid from behind the primary seal and on to the outside pistons of the rear caliper. (That primary seal yields easily when pressure comes from behind - a bit like the fibre cup inside an ordinary bike pump - when pushing the pump air is forced into the tyre, on the upstroke the same fibre cup gives way and allows air from behind to pass, ready to be pushed into the tyre on the next stroke.)
You can prove that fluid passes through the SMC in this manner by opening the rear outer bleed valve and pressing the brake pedal. Fluid comes out easily.
Here is a diagram of the full circuit with the line from the brake pedal to the SMC in dark purple, and the continuation through the SMC to the rear caliper in a lighter purple.
Ok - so your bike is on the centre stand, you press the brake pedal and pressure is applied to all 3 pistons in the rear caliper.
When you release the pedal, the pressure should be able to return to the master cylinder reservoir through the tiny compensation port that is uncovered when the primary seal in the master cylinder moves back past it. Unfortunately, if there is damage to either master cylinder that is preventing the full return of the piston to its relaxed state, the compensation port remains blocked and all of that pressure that you applied to the rear brake outer pistons can not escape. So your brakes do not release.
This applies to all master cylinders - at the clutch lever, the brake pedal, the front brake lever and the secondary master cylinder.
Unfortunately it is the SMC that is likely to get the most blame. It is the only one that has an upward pointing plunger, and a well which is able to catch and retain the water. It is the only one that has a dead spot in the fluid which requires the plunger to be activated and tilted to allow all of the air and all of the old water soaked fluid to be flushed out, and it is the only one where a small knock is likely to bash the master cylinder casing and 'put an egg in it' - ie make it slightly oval, impeding the full movement of the piston inside.
Here is one such SMC that failed - sawn lengthways down the cylinder bore. It appears to get narrower at the left hand end because I had to make the cut at a slight angle.
The red marks show the position of the inlet port (left) and the compensation port (right). Not that the piston only has to move a small amount before it passes and seals off the compenstation port. Ithe hole is directly in front of the lip of the primary seal. (The primary seal is the one that applies the pressure - the one just to the left of the spring).
The same SMC with the piston removed, showing the inlet port on the left and the oh-so-tiny compensation port on the right - circled.
If fluid is not flushed through properly when the brake fluid is renewed, this port can also become clogged with gunge - it eventually crystallises.
I change my fluid every year or so. I never wait the full two years.
Note also the amount of muck on the bore at the left hand end - due to water ingress and corossion. This build up prevents the piston from returning properly and leaves the primary seal blocking the compensation port.
Links to 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
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
A faulty SMC, taken apart, sawn lengthways, with labelled photos and a commentary. This SMC had a brand new service kit fitted not long before it failed again. A while ago I posted a query concerning some weird behaviour on a friends braking system. The symptoms were that in spite of getting...
www.st-owners.com
and I've ordered a new SMC.
