I have the rear brake caliper mounting brackets on the bench at the moment and the guide pins appear to be in very good working order. They slide easily and the grease appears clean.
This leaves me wondering why the inboard pads have worn 27% faster than the outboard pads.
If the caliper piston were not retracting (slightly) wouldn't wear be equal on both pads?
Or were the pads were sticking in the guide shims? But they look OK. Anyway - both pads wearing on the inboard only, and by exactly the same amount?
I noticed when inspecting the guides that if I pushed the pin inward air was expelled from the boot which then held the pin in by a partial vacuum, because the air pressure only increases the sealing effectiveness. On the other hand if the pin is pulled outward there is a force acting to push the pin inward which would result in a small constant push of the inboard pad onto the disc, because it is trapped between the caliper piston and the disc.
This led to the idea that the grease inside the guide would act as a seal to prevent movement of air around the pin, resulting in even stronger forces, one of which is in the same direction required to press the pad to the disc . . . but only if the guide was packed fully with grease, which it wasn't.
So in conclusion: Unresolved. The brakes have been great up to now though.
This little animation shows the action of a single piston sliding caliper:
https://www.youtube.com/watch?v=ar5j1mUdFA8
The guide pins on the Mondeo are different though, effectively reversed with the pin sliding into a hole blocked at one end.
This one is more for entertainment:
https://www.youtube.com/watch?v=YJmDNJn0WW0