Abstract: A pressure intensifier comprising a housing incorporating a first floating piston-cylinder unit with means to supply a primary pressure fluid medium to one side of the piston to apply pressure to a fluid medium in contact with the other side of the first piston, a second floating piston unit provided with a fluid medium communication port therethrough, there being direct communication between the medium contacting the other side of the first floating piston with one side of the second floating piston and through the port thereof to a fluid medium pressure power take-off chamber, the port of the second floating piston being closeable by means of a valve comprising two active parts, one part of which is incorporated in the second floating piston and the other part of which is provided with limited movement with respect thereto, there being means to apply primary fluid pressure to the other side of the said second floating piston to maintain the communication port open, the other part of the valve being constitu