U should be able to move the plate ont top with a pair of mole grips and should phisically see the piston move and grip the disk . There a fooker to put back together without another set of hands but do able . Technically yes you should change the seals and there not to expensive so it makes sence . But as Dojj said check your hand brake cable is adjusted up correctly and that when the handbrake is applied its not slack on the side the side thats inop