Hope changed piston material from metal to phenolic (kind of plastic) to reduce seizing.
The brakes I've had that seized and that I couldn't fix by removing the pads, dripping DOT oil on the pistons and "massaging" back and forth until they ran smoothly all turned out to have damaged (worn) pistons.
If they are monos, and without a compressor they are a nightmare to strip down.
Completed: 94 Stumpjumper w. M733; ; 92 Dave Yates 731 ; 97 Parkpre Pro-Elite ; Pace RC200 F6 ; Pace RC300; 82 Gios Super Record ; 97-8 De Rosa Giro D´Italia; ; Banani SPX Track bike ; 89 Alan/Mavic Cross Bike ; 05 Focus Cross Team ; 90 Look MI-80 all-Mavic ; 12 Focus Izalco Team/custom build ; 11 Focus Thunder Custom build
Projects : , Brouwer all-Mavic
Stripped : Pace RC303
Gone : GT Karakoram
Stayed in the family: Kuota Kharma, 04 Stumpjumper FSR
Ceased to be : Dynatech MT3, killed by corroded seatstays.