The secondary master cylinder (SMC) that is being referenced is attached to the left front caliper assembly on the left fork leg. It is operated by being forced to rotate upward and forward on its lower pivot point by being dragged by the rotational force of the left front brake disc when the brakes are applied. This causes it to apply hydraulic pressure to the rear caliper to apply the rear brakes. If this SMC does not fully release the rear brake pressure is not fully released and the rear brakes do not fully release. This is a common failure with ST1300's brakes that have been poorly maintained.
There are two quick simple tests that you can do to help diagnose it.
First test- With the motorcycle on the centre-stand the rear wheel should rotate freely.
- Manually push the SMC upwards and forward towards the fork leg rotating it on its lower pivot point. It does not move very much at all- only a few millimetres so don't expect a lot of movement. This actions applies the rear brake and prevents the rear wheel from rotating. You can have someone help you or you can lie down along the left-hand side of the motorcycle and try to rotate the rear wheel with your foot while roatating the SMC. Be mindful not to push the motorcycle forward off of the centre-stand while applying force to the SMC. Chocking the front wheel is not a bad idea. However you accomplish this the point is that rotating the SMC forward should apply hydraulic pressure to the rear brake and lock the rear wheel preventing it from rotating.
- Release the SMC and the rear wheel should immediately be free to rotate again. If it isn't the SMC is most likely seizing up and not releasing the hydraulic pressure to the rear wheel.
Second test- Again, with the motorcycle on the centre-stand.
Without applying any force to the SMC the rear wheel should rotate freely. If it doesn't open the rear caliper bleeder nipples one at a time to release the hydraulic pressure taking note of which bleeder allowed the rear wheel to rotate when it was opened. Whichever bleeder nipple it was that allowed movement will determine which caliper pistons are sticking and will further help determine whether the problem is a seizing SMC, hydraulic caused by some other problem maintaing pressure, or mechanical due to one or more of the pistons sticking or some other mechanical default.
If opening the bleeder screws at the rear caliper does not allow the rear wheel to rotate freely than the problem is most likely not hydraulic but mechanical. The above tests are intended to be an indicator of where to focus further inspection. They are not definitive as the entire brake system operates as one system and all parts of it need to function properly and as such the entire brake system needs to be inspected and serviced.
The below is an excellent article that will show you what to look for and how to inspect it.
