Check the disk surface. They will groan if the surface has grown some rust overnight. It clears within a few operations though.
Do you have the chrome backing plates fitted, and the white heat shields ?
It is possible that the pads are not sitting parallel to the disc surface. Check your pad spring is fitted the correct way round - you will need to remove the pads.
The wider strip of the spring should be closer to the pistons. But note the two little tags on the narrow strip. The inner pad backing plate has to fit behind those tags. Fit the spring pressing it to clip into the roof. Put in the outer pads, and insert the clean, smooth pad pin part way to hold the pad. Partly insert the inner pad and pull the caliper to lightly trap it against the disc surface. Rotate the wheel to take the inner pad with it. By trapping the pad against the disc, it makes sure the pad fits the correct side of those tags. Push the pad pin through to hold the pad. Make sure it is seated in the chrome retaining clip - look through the wheel spokes from the right.
What make of pads do you have ?
Take a look at this for some pics.
https://www.st-owners.com/forums/threads/st1300-maintenance-brakes-avoiding-the-pitfalls.135125/
That is step one.
If it doesn't fix it, report back, we might need to look at how the caliper bracket is seating.
It would help if you noted any uneven wear on the pads.
