If the tire tech did not insert and tighten the retaining bolt in the fork leg lug the clevis probably came detached under braking and broke off and/or levered the retaining circlip in the SMC bore out of place thus releasing the piston and spring.
This is a very plausible scenario, especially if it can be determined that the SMC clevis is no longer attached to the lug on the left fork leg.
On the other hand, what would be a plausible scenario if the SMC clevis is discovered to be still firmly attached to the left fork leg? I'll venture an idea.
As the Honda ST1300 does have a history of contamination entering into the SMC at the cylinder opening, and causing corrosion to develop in that area:
- The corrosion can eventually lead to a failure of the rear brake caliper not releasing properly.
- A non-releasing rear brake caliper will create massive amounts of heat, that will damage the brake rotor, the brake pads, and cause the brake fluid to boil inside the brake caliper.
- The effect of the boiling brake fluid would appear quite differently on the two rear brake caliper circuits.
- The rear master cylinder brake fluid circuit would probably experience a loss of pressure when the brake pedal is depressed, due to brake fluid vaporization. The expanding brake fluid (due to heat) would merely pass into the rear master cylinder reservoir, with no increase in brake line fluid pressure.
- The SMC brake fluid circuit would experience a build up of pressure (due to heat), and with the path to relieve pressure blocked (corrosion of the SMC bore), it is possible the accumulated pressure could force the SMC piston out of the cylinder at a high rate of speed.
I think that a close examination of the condition of the circlip retaining boss on the SMC, as well as the condition of the rear brake rotor (for signs of over-heating) is warranted. If the SMC piston was forced out of the SMC due to high pressure, I would seriously consider replacing the entire SMC unit as the internal plastic check valve may have been compromised. Also I think the replacement of the rear brake pads should be considered.