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 .