This is a very old brake and maybe not frequently serviced?
If so, the piston rubbers might be sticky causing the piston to stick. The best solution will be to open the system and clean all parts. if this is a DOT brake there's a chance for dirt in the O-ring housings (pistonrubbers), we call it coliflower in the workshop. Best to remove this is by scratching with a thin screwdriver and remove all the dirt. I also like to leave the O-rings in a bath of teflonspray, the same one that you use for the chain. This will bring the old rubber back to life. After a day you can check the O-ring on cracks, this is better to see after the treatment than before.
If you don't know how to deal with these brakes you can always ask a motormech, they should understand these brakes since Yamaha used one back in the 80's (if I'm correct model TR1).
Is this the Fisher that was for sale on eBay a few days ago? Or did you borrow the pic?