A working caliper piston is only part of story to having functional brakes.
Of equal importance is that the pads are slightly loose on the seat of the carrier, something like 0.5 - 0.75mm, which allows to the pad to expand with heat without binding. Copper grease on the locating tips is a must.
Carrier must be scraped clean of rust to bare metal also.
If they have stainless metal formed shims the rust forms unseen below them.
The sliding mechanism pins are next, these need cleaned and greased with silicone grease, they should be nice and free but not sloppy axially.
Next, fluid must be in good condition as it's hygroscopic and if this simple maintenance item is ignored you will get gunk and rust in the lines which contribute enormously to stickiness of the piston.
Lastly, and often overlooked are the flexi lines, for the cost of them, and on these heavy machines and at the age yours is at, get em changed, you'd be amazed how badly they suffer internally from dizzy mechs clamping them with the wrong tools.
You have Brembo fronts, but standard rears if I recall correctly.
EDIT!! Your front Brembo calipers are of fixed body design, they have no sliding pins. (other than the pad retaining ones at the front)
