a lockable hydraulic plunger similar to car hydraulic hand brakes.
or you could incorporate a brake locking device from a big quad..
or even a 12volt switchable relay and hydraulic plunger...
search Only for local ''scrap'' yards,most big company's will scrap unwanted machinery from production lines...i spend many and hour sniffing round looking at and in these machines..last time i found 12v solinoids capable of holding back 3000psi,i used them in a nitrous oxide NO2 bike i built..