...is it the SMC? It’s only a year old.
It could be the Secondary Master Cylinder, for reasons John has explained in his post above (reasons that are fully elaborated in the two scholarly articles that he posted links to), but then again, it could be a few other things, some of which are quite simple to solve.
The brake calipers on the ST are 'fully floating', in the sense that the piston only works on one of the two brake pads, and the entire caliper assembly slides laterally to apply equal pressure to the brake pad on the other side of the caliper. To work properly - and in your case, to release properly - the entire caliper assembly has to be free to slide back and forth on two pins that hold each caliper in place. Different problems can cause the calipers to not be able to slide freely, for example, lack of lubrication on the pins, dirt and crud getting into the pin assembly because rubber boots around the pins have deteriorated, or improper (usually, out-of-sequence) installation or re-assembly of the calipers.
So, I think you should first go looking for "mechanical" causes (calipers not sliding freely) before you go looking for hydraulic problems.
Along a similar "mechanical problem" line of thought, as John noted above, it is possible to get the position of the upper tang of the brake pads wrong when installing the pads, this because a little spring-like contraption at the upper end of the brake pad slips out of place when doing the re-assembly. If this happens, the pad will not be parallel to the caliper, and will not properly release when you release brake pressure. If left like this, the pad will wear unevenly.
Yet another possible mechanical problem is the free movement (pivoting) of the SMC about its pivot point. In this case, I'm not referring to how the internals of the SMC work, or whether they work or not, I'm referring to whether or not the SMC freely articulates back and forth. Improper assembly, lack of lubrication, too much torque applied to fasteners, deterioration of bearings and bushings, or damage to the fibre washer that is glued (by Honda) onto the front left fork where the SMC attaches can all impede smooth movement of the SMC body.
A final possible "mechanical" problem - although I suppose this also falls into the "hydraulic" category - is that one or more of the pistons in a caliper is binding and not free to fully release because of dirt and crud that has accumulated around the outer edge of the piston where it retracts into the caliper in the absence of pressure. Here's a couple of links that discuss solving stuck piston and stuck (non-floating) calipers, and how to clean those parts to solve that problem:
Left Front Caliper Slider Pins and
New OEM pads on the rear problem. Larry's (
@Igofar) trick of using a flat shoelace soaked in brake fluid to clean the outside of the three pistons in each caliper works really well, and is cheap to implement.
Do you have a Honda Service Manual? Although information posted here on the forum is a superb source of information about how things work, I have found that the instructions contained in the Honda Service Manual for assembly and disassembly of the brake components are very detailed, and I have also discovered (the hard way) that it is essential to follow the instructions - particularly the sequence of doing work - in the manual when servicing the brake system.
Michael
the brake cleaner spray may/could/and often will do more damage than good by swelling the o-rings and causing the pistons to drag. Not to mention, it will completely dry everything out....you guessed it, causing the pistons to drag 
or stop by and let Tom take a peek at it.
