Ditto what Larry said. Don't buy a rebuild kit. Buy the full SMC assembly. The second photo above - the cutaway - contains a rebuild kit that I had in stock. I gave it to a friend who was having trouble with his SMC. It didn't cure the problem that he had (I had just bought a 2009 model so I couldn't use it), and he gave me the entire assembly less than 2 years later. The problem was with the SMC bore not the piston assembly.
I cut it down the length of the bore as you can see.
I checked if you were in the UK - by looking up Basingstoke in the USA. Yes - there is one there as well, so I wasn't sure.
All suppliers of Honda motorcycle parts seem to be tied to the same controls - and every single outlet quotes the same price. Don't bother looking for cheaper, the websites show prices within pence of each other. Some will have more postage and packing. Some will not show VAT.
The biggest outlet that I know of in the UK is Fowlers Parts
Fowlers Parts | Genuine Motorcycle Parts & Spares
They are no cheaper or dearer than anywhere else, but they carry a huge stock and are very fast.
The model changed in 2008. Make sure when you order that you use the model year of your bike - not the registration year.
The model year is on the white label under the pillion seat. It will say ST1300A6 or ST1300A8 or ST1300-4 or something similar. The ST1300A6 is a 2006 model with ABS. The ST1300-4 is a 2004 model without ABS.
So what year model is your bike ? This may be different from the registration plate.
If there is no white label, look at the Vehicle ID on the right hand side of the headstock. That will probably start with JH2SC5 - or something similar. The 10th character is the model year. It is the same coding 6=2006, 8=2008, E=2014. (There wasn't a 2005 model or a 2007 model in the UK, and there were no new models between 2009 and 2014).
Just glanced on Fowlers website for two types. You would need to check these numbers for yourself. You need to specify the make, model and year of the bike.
Example.
If it is a 2007 registered bike, with ABS and the model year is 2006, then you would specify
Model ST1300A / Year 2006 (not 2007).
They are shown in the
L. Front Brake Caliper Fiche.
Just checked fowlers site.
Pre 2008 is 06454-MCS-G03 - in stock. £215.72
2008 and later is 06454-MCS-R02 -
not in stock £215.72
Another one that I have used is
https://www.lingshondaparts.com
For that, you need the bike serial number to choose the correct fiche - although all versions of bikes use one of the two types above.
Lings don't list in stock or out of stock, so you'd have to phone.