A fluid circuit assembly for maintaining substantially constant pressure between two abutting solid objects, the circuit assembly comprising a master cylinder assembly, a plurality of working cylinder assemblies, piston rods extending from the working cylinder assemblies, a spool on distal ends of the piston rods adapted to engage a surface against which it is desired to maintain constant pressure, the master cylinder and the working cylinders having first chambers in communication with a first fluid circuit, the master cylinder having a second chamber in communication with a source of gas under a selected substantially constant pressure, said gas being in communication with a gas reservoir portion of a liquid/gas tank, a liquid reservoir portion of said tank being in communication with second chambers of said working cylinder assemblies, such that upon change of pressure on the spool by the surface, a reaction is generated in the circuit assembly, such as to move the spool to restore a selected pressure between the spool and the surface.