this picture makes it look like its part of the casting, but its a removable assembly
That's correct, the inlet-port check-valve assembly is a removable piece.
The purpose of this sketch (perhaps schematic would have been a better description) was to map out the hydraulic 3D pathways of the SMC, such that they would fit on a single sheet of paper.
Just a cautionary note that referring to this valve as a "compensating valve" may lead to some unintentional confusion with the compensating port of the SMC.
I think the purpose of this valve is simply to allow brake fluid a high volumn flow from the rear master cylinder to both the SMC, and the rear brake caliper, while also restricting the flow of returning brake fluid back to the rear master cylinder.
Thus my referencing the valve as a "check valve" associated with the brake fluid "inlet-port".
This issue can be fixed without buying any parts. The brake piston rubbers were just fine, no problems in the bore. The issue was the gooey gelled brake fluid in the SMC and most importantly, up in the compensating chamber of the SMC.
This is pretty much the same scenario that I experienced the first time the rear brakes on my ST1300 were dragging (they hadn't yet gotten to the full "locked-up" phase), a good cleaning, and reassembly of the old seals was all that was required to be back on the road.
Here's a brief SMC history for my 2006 ST1300/A:
- At 8,622 miles, cleaned & rebuilt (with original seals) the SMC.
- At 56,179 miles, replaced the SMC with a new SMC.
- At 56,999 miles, replaced the SMC (new) with the original SMC (rebuilt with new seals), and packed the push rod bootie with silicone grease.
- Currently at 109,365 miles, SMC is functioning properly.
Glad you've gotten to the cause of the problem, were able to correct it, and now you're back on the road.
Congratulations.