Hi Henry:
It's an honour to help - we are a very friendly community here, and our primary focus is on helping each other with technical issues (although some might argue that our primary focus is on finding interesting places to ride to and eat at
).
I suggest you start by downloading and printing out, then carefully reviewing, the very scholarly article that John Heath (
@jfheath ) wrote explaining ST 1300 brake system maintenance. It's attached to the first post of this discussion:
ST1300 Maintenance - Brakes- Avoiding the Pitfalls. I suggest printing it and reading it from the paper copy because the brake system is complex, and you will likely want to refer back to that document many times as you read other discussions about the brake system.
The next article I suggest you read is the first post on the discussion
Brakes - Operation of the Master Cylinder. That will give you a solid background on how the master cylinder works, although I don't think that the master cylinder is the cause of your problem. Nevertheless, when troubleshooting brake issues, one needs to have a good grasp of the concepts of how everything in the system works, and that post about how the master cylinder works explains things really well.
Next, go have a look at some of the discussions about the Secondary Master Cylinder (SMC), which is an assembly up on the left side of the front wheel that is critically important to correct operation of the rear brake. It's premature to speculate that your problem is caused by that part, but once again, you need to know how the system works, and that particular part is often the cause of problems - not necessarily because the part fails, but because folks fail to exactly follow the shop manual instructions when replacing brake fluid or replacing brake pads. Here are two good posts to review (don't get too caught up in the details, just try and get comfortable with the purpose of the Secondary Master Cylinder):
ST1300 Maintenance - Photos of dismantled SMC and also
The inner workings of the Secondary Master Cylinder (SMC).
Finally, take a close look at this article:
ST1300 Maintenance - Getting all the air out of the brake system. There is a PDF attached at the bottom of the first post, download that PDF to enable you to see the diagrams at full size. The real value of that post is not learning how to get air out of the system (although that certainly helps solve problems), it is the diagrams that show you the very complex path that the brake fluid takes, and the different ways that both front and rear brakes can be applied. For example, did you know that every time you pull the front brake handle, you also apply the rear brake? Likewise, every time you press down on the rear brake pedal, you also apply the front brake.
By the time you get through all that reading, you will most likely have a much better understanding of the ST 1300 brake system than most Honda service technicians.
Michael