Parallel Fourier-Motzkin Elimination

In the illustrative embodiments disclosed, an improved pressure-adjustment valve assembly for a pneumatic spring includes a valve channel connected at one end to the interior and at the other end to the exterior of the spring cylinder. The valve channel is preferably defined in part by a bore in an integral part of the cylinder and in part by a cap member removably mounted on such integral part. A valve body is axially movable within the valve channel between open and closed positions, coacting when in the closed position with a surrounding sealing ring to seal the cylinder against fluid leakage. The valve body is shaped to insure long seal lifetime. All components of the valve structure are designed and constructed to facilitate manufacture, use and assembly of the valve and to permit use of readily available materials.