A prosthetic lock assembly comprising: a lock body defining a pin hole adapted to receive an attachment pin, the lock body further defining a radial bore intersecting the pin hole, and a chamber carrying a locking device that selectively locks on the attachment pin and fluidly connects the pin hole and the radial bore to unidirectionally lock the attachment pin in the lock body while permitting insertion of the attachment pin in the lock body, wherein the locking device is positioned in the chamber defined by the lock body; and a valve assembly positioned in the lock body and fluidly connecting to the chamber, the valve assembly moveable between a closed position in which the valve assembly is sealed, and an open position in which the valve assembly is unsealed so that air moves into the lock body via the valve assembly; wherein the valve assembly includes a pin member arranged to selectively engage with the locking device and move the valve assembly between the open and closed positions; wherein the valve assembly includes a spring member adapted to bias the valve assembly toward the closed position, the spring member located on the pin member; the prosthetic lock assembly further comprising a release mechanism positioned in the radial bore on a side of the locking device in the chamber opposite of the valve assembly, the release mechanism arranged to both unlock the attachment pin from the lock body and move the valve assembly to the open position by translating the locking device in the lock body; in that the release mechanism is arranged to be pushed into the lock body to urge the locking device in a first direction and disengage a pin engagement part of the lock body; characterized in that the locking device is arranged to be urged toward the valve assembly by the release mechanism; in that the prosthetic lock assembly further comprises supplementary spring members positioned between the locking device and the lock body in the chamber, the supplementary spring members arranged to bias the locking device away from the valve assembly, wherein when the attachment pin is removed from the lock body and the release mechanism is released, stored energy in the supplementary spring members forces the locking device back in a second direction opposite the first direction, returning the prosthetic lock assembly toward an unloaded position, and simultaneously, stored energy in the spring member on the pin member forces the valve assembly toward the closed position, thereby resealing the chamber; and in that it comprises at least one one-way valve (768;770) positioned in the lock body and arranged to allow air to exit the chamber via the at least one-way valve. a lock body defining a pin hole adapted to receive an attachment pin, the lock body further defining a radial bore intersecting the pin hole, and a chamber carrying a locking device that selectively locks on the attachment pin and fluidly connects the pin hole and the radial bore to unidirectionally lock the attachment pin in the lock body while permitting insertion of the attachment pin in the lock body, wherein the locking device is positioned in the chamber defined by the lock body; and a valve assembly positioned in the lock body and fluidly connecting to the chamber, the valve assembly moveable between a closed position in which the valve assembly is sealed, and an open position in which the valve assembly is unsealed so that air moves into the lock body via the valve assembly; wherein the valve assembly includes a pin member arranged to selectively engage with the locking device and move the valve assembly between the open and closed positions; wherein the valve assembly includes a spring member adapted to bias the valve assembly toward the closed position, the spring member located on the pin member; the prosthetic lock assembly further comprising a release mechanism positioned in the radial bore on a side of the locking device in the chamber opposite of the valve assembly, the release mechanism arranged to both unlock the attachment pin from the lock body and move the valve assembly to the open position by translating the locking device in the lock body; in that the release mechanism is arranged to be pushed into the lock body to urge the locking device in a first direction and disengage a pin engagement part of the lock body; characterized in that the locking device is arranged to be urged toward the valve assembly by the release mechanism; in that the prosthetic lock assembly further comprises supplementary spring members positioned between the locking device and the lock body in the chamber, the supplementary spring members arranged to bias the locking device away from the valve assembly, wherein when the attachment pin is removed from the lock body and the release mechanism is released, stored energy in the supplementary spring members forces the locking device back in a second direction opposite the first direction, returning the prosthetic lock assembly toward an unloaded position, and simultaneously, stored energy in the spring member on the pin member forces the valve assembly toward the closed position, thereby resealing the chamber; and in that it comprises at least one one-way valve (768;770) positioned in the lock body and arranged to allow air to exit the chamber via the at least one-way valve.