A pneumatic or hydraulic mechanism has a housing defining a piston chamber and having a fluid inlet port. A piston is slidable in the piston chamber. The piston partitions the piston chamber into a front chamber and a rear chamber. The piston has one or more passages for fluid communication between the rear chamber and the front chamber, the one or more passages being sealed by a sealing mechanism. The sealing mechanism has a sealing state in which the sealing mechanism substantially inhibits fluid communication between the rear chamber and the front chamber, and a non-sealing state in which the sealing mechanism allows fluid communication between the rear chamber and the front chamber. The piston is slidable between a first position and a second position. When the piston is positioned in the first position, the sealing mechanism is in the sealing state. Upon supply of a fluid to the inlet port, the fluid urges the piston to its second position and then causes the sealing mechanism to change to the non-sealing state until the pressure in the rear chamber and the front chamber equalises, allowing the sealing mechanism to return to the sealing state. Upon removal of fluid from the rear chamber, the fluid in the front chamber urges the piston to return to its first position.