dust boots dry up and or fail because of heat . . created by dragging pads over time . .
The inner seal, which is a square cross section o-ring has lost it's resiliency. This o-ring twists a tiny bit when the brakes are applied, and returns to it's neutral sate when you release pressure. This releasing of pressure causres the square o-ring to move the piston back in the bore.
My advice, take them all a part and clean and rebuild properly!
the kit will include all the pieces necesary, the bleeders are available separately. We always replace these in the process as the 8mm hex gets worn away.